drm/i915: Extract intel_panel_encoder_fixed_mode()

Apart from the EDID and VBT based mechanism we also sometimes
use the encoder's current mode as the panel fixed mode. We
currently have the same code for that duplicated in two places.
Let's unify.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220323182935.4701-8-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>