drm/modes: Move 3D stereo flag check into drm_mode_validate_basic()

drm/modes: Move 3D stereo flag check into drm_mode_validate_basic()

Currently we don't sanity check the 3D stereo flags for modes filled out
by the kernel. Move the check from drm_mode_convert_umode() into
drm_mode_validate_basic() so that we get the same check going both ways.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114183258.16976-2-ville.syrjala@linux.intel.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>