drm/i915/display: move to_intel_frontbuffer to header

This will be used for some refactoring in other files, so move it
first.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201221090449.8288-2-jani.nikula@intel.com