drm/i915/hdcp: Check if media_gt exists

Check if media_gt exists if we are using gsc cs

--v2
-correct typo [Ankit]
-assign gsc variable if gt exists [Ankit]

--v3
-declare gsc and gt variables in if block [Ankit]

--v4
-add fixes tag [Ankit]

Fixes: 88363177 ("drm/i915/mtl: Add HDCP GSC interface")
Cc: Tomas Winkler <tomas.winkler@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230505041512.585486-1-suraj.kandpal@intel.com