drm/i915/display: New function to avoid duplicate code in upcomming commits

Moves some checks that later will be performed 2 times to an own function.
This avoids duplicate code later on.

Signed-off-by: Werner Sembach <wse@tuxedocomputers.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210510133349.14491-2-wse@tuxedocomputers.com