drm/i915/dp: Restructure the sink/output format selection

drm/i915/dp: Restructure the sink/output format selection

Restructure intel_dp_compute_output_format() to resemble the new
intel_hdmi_compute_output_formats().

Again, we basically have two main code paths:
- YCbCr 4:2:0 only modes
- everything else including YCbCr 4:2:0 also modes

Take the exact same approach with the DP code, making the
format selection much less convoluted.

Cc: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260409101539.22032-5-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Tested-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>