drm: Reorganize probed mode validation

Make drm_mode_validate_size() and drm_mode_validate_flag() deal with a
single mode instead of having each iterate through the mode list.

The hope is that in the future we might be able to share various mode
validation functions between modeset and get_modes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>