drm/probe-helper: Call drm_mode_validate_ycbcr420() before connector->mode_valid()

drm/probe-helper: Call drm_mode_validate_ycbcr420() before connector->mode_valid()

Make life easier for drivers by filtering out unwanted YCbCr 4:2:0
only modes prior to calling the connector->mode_valid() hook.
Currently drivers will still see YCbCr 4:2:0 only modes in said
hook, which will likely come as a suprise when the driver has
declared no support for such modes (via setting
connector->ycbcr_420_allowed to false).

Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/10992
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240516173324.18149-1-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>