drm/i915/fbc: Nuke state_cache

fbc->state_cache has now become useless. We can simply update
the reg params directly from the plane/crtc states during
__intel_fbc_enable().
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211124113652.22090-17-ville.syrjala@linux.intel.comReviewed-by: Mika Kahola <mika.kahola@intel.com>