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)