drm/i915: Shuffle DP .mode_valid() checks

Move some of the more trivial checks in the DP .mode_valid()
hooks upwards to lessen the noise amongst the more complex
checks.

Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240402135148.23011-3-ville.syrjala@linux.intel.com