drm/i915/intel_fb: Pull is_surface_linear() from intel_display.c/skl_universal_plane.c

Move is_surface_linear() to intel_fb.c and export it from here, also
removing the duplicate definitions of it.

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/20210325214808.2071517-9-imre.deak@intel.com