drm/i915/display: Use a sub-struct for fbc operations in intel_display

drm/i915/display: Use a sub-struct for fbc operations in intel_display

As FBC can utilize the system cache in xe3p_lpd onwards, we need
a provision to track which fbc instance is utilizing this cache.
A sub-struct at intel_display level to group all the fbc ops will
make fbc handling much easier. Introduce a fbc sub-struct and move
the fbc instance array into that.

v2: changes in commit message

Suggested-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patch.msgid.link/20251127115349.249120-2-vinod.govindapillai@intel.com