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

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

BugLink: https://bugs.launchpad.net/bugs/1934489
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
(backported from commit eacba74d4d561ea6487d944417526e1b025cbebd)
Signed-off-by: Werner Sembach <wse@tuxedocomputers.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>