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

Move the FB plane specific function from intel_sprite.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/20210325214808.2071517-10-imre.deak@intel.com