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

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)

Fixes: 4af50beb4e0f ("drm/xe: Use gsc_proxy_init_done to check proxy status")
Cc: <stable@vger.kernel.org> # v6.10+
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Shuicheng Lin <shuicheng.lin@intel.com>
Link: https://patch.msgid.link/20260416-check-for-null-media_gt-in-intel_hdcp_gsc_check_status-v2-1-9adb9fd3b621@intel.com
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>