### Step 1: Root Cause Analysis

1. **Understanding the Code Context:**
   - The function `i915_gem_create` is responsible for creating a GEM (Graphics Execution Manager) object.
   - The `size` parameter represents the size of the object to be created.
   - The `roundup` function is used to align the `size` to the nearest page boundary.
   - The `i915_gem_alloc_object` function allocates memory for the GEM object based on the given `size`.

2. **Identifying the Vulnerability:**
   - The vulnerability is identified at line 7: `obj = i915_gem_alloc_object(dev, size);`.
   - If `size` is zero, the `i915_gem_alloc_object` function might not handle this case properly, leading to undefined behavior or an out-of-bounds write.

3. **Potential Issues with Zero Size:**
   - If `size` is zero, the `roundup` function might still return zero.
   - Allocating an object with zero size can lead to unexpected behavior, such as writing to an invalid memory location or causing a buffer overflow.

### Step 2: Fixing Strategy

1. **Adding a Check for Zero Size:**
   - To prevent the allocation of an object with zero size, we need to add a check after the `roundup` function.
   - If `size` is zero, the function should return an error code to indicate invalid input.

2. **Implementing the Fix:**
   - After rounding up the `size`, check if `size` is zero.
   - If `size` is zero, return an error code (e.g., `-EINVAL`).

### Conclusion

The root cause of the vulnerability is the potential for `size` to be zero, leading to an invalid memory allocation. The fix involves adding a check to ensure `size` is not zero before proceeding with the memory allocation.

### Patch Implementation

The patch ensures that the function returns an error if `size` is zero, preventing the allocation of an object with zero size and avoiding the out-of-bounds write vulnerability.

```diff
<     size = roundup(size, PAGE_SIZE);
<     obj = i915_gem_alloc_object(dev, size);
---
>     size = roundup(size, PAGE_SIZE);
>     if (size == 0)
>     {
>         return -EINVAL;
>     }
>     obj = i915_gem_alloc_object(dev, size);
```

### Reasoning Steps for Generating the Patch

1. **Identify the Vulnerability:**
   - Recognize that allocating an object with zero size can lead to undefined behavior.

2. **Determine the Fix:**
   - Add a check to ensure `size` is not zero after rounding up.

3. **Implement the Fix:**
   - Insert the check and return an error code if `size` is zero.

By following these steps, the patch effectively mitigates the CWE-787 vulnerability by ensuring that the `size` is valid before proceeding with the memory allocation.