drm/xe/hdcp: Add NULL check for media_gt in intel_hdcp_gsc_check_status()

In the Linux kernel, the following vulnerability has been resolved: drm/xe/hdcp: Add NULL check for media_gt in intel_hdcp_gsc_check_status() When media GT is disabled via configfs, there is no allocation for media_gt, which is kept as NULL. In such scenario, intel_hdcp_gsc_check_status() results in a kernel pagefault error due to &gt->uc.gsc being evaluated as an invalid memory address. Fix that by introducing a NULL check on media_gt and bailing out early if so. While at it, also drop the NULL check for gsc, since it can't be NULL if media_gt is not NULL. v2: - Get address for gsc only after checking that gt is not NULL. (Shuicheng) - Drop the NULL check for gsc. (Shuicheng) v3: - Add "Fixes" and "Cc: <stable...>" tags. (Matt) (cherry picked from commit bfaf87e84ca3ca3f6e275f9ae56da47a8b55ffd1)