drm/i915/fbc: Track FBC usage per-plane

In the future we may have multiple planes on the same pipe
capable of using FBC. Prepare for that by tracking FBC usage
per-plane rather than per-crtc.

v2: s/intel_get_crtc_for_pipe/intel_crtc_for_pipe/

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211124113652.22090-9-ville.syrjala@linux.intel.com


Reviewed-by: Mika Kahola <mika.kahola@intel.com>