drm/i915/intel_fb: Pull FB plane functions from skl_universal_plane.c

Move the FB plane related functions from skl_universal_plane.c to
intel_fb.c.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210325214808.2071517-8-imre.deak@intel.com