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

Move the FB plane specific functions from intel_display.c to intel_fb.c.
There's more functions like this, but I leave moving those as well for a
follow up, and for now moving only the ones needed by the end of this
patchset (adding support for padding tile-rows in an FB GGTT view).

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/20210325214808.2071517-11-imre.deak@intel.com