drm/i915/fbc: Reuse the same struct for the cache and params

The FBC state cache and params are now nearly identical. Just
use the same structure for both.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211124113652.22090-7-ville.syrjala@linux.intel.com


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