Root Cause Analysis: Based on the provided code and the summary of the `i915_gem_alloc_object` function, the potential CWE-787 (Out-of-bounds Write) vulnerability exists because the `size` parameter passed to `i915_gem_alloc_object` is not validated or sanitized before being used to allocate memory for the `drm_i915_gem_object` structure.

The vulnerability lies in the fact that the `size` parameter is directly used to allocate memory without any checks or bounds validation. If an attacker can control or manipulate the `size` parameter, they could potentially cause an out-of-bounds write by providing a large or negative value, leading to a buffer overflow or memory corruption.

To mitigate this vulnerability, the code should perform proper input validation and sanitization on the `size` parameter before passing it to `i915_gem_alloc_object`. This could include:

1. Checking if the `size` is within a reasonable range (e.g., not negative or excessively large).
2. Ensuring that the `size` does not exceed a predefined maximum value or the available memory resources.
3. Sanitizing the `size` value to prevent integer overflows or other arithmetic vulnerabilities.

By implementing these checks and validations, the code can prevent an attacker from exploiting the vulnerability and causing an out-of-bounds write or buffer overflow.