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.comReviewed-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/drm_modes.c

@@ -1036,6 +1036,9 @@ EXPORT_SYMBOL(drm_mode_equal_no_clocks_no_stereo);
 enum drm_mode_status
 drm_mode_validate_basic(const struct drm_display_mode *mode)
 {
+	if ((mode->flags & DRM_MODE_FLAG_3D_MASK) > DRM_MODE_FLAG_3D_MAX)
+		return MODE_BAD;
+
 	if (mode->clock == 0)
 		return MODE_CLOCK_LOW;
 
@@ -1574,9 +1577,6 @@ int drm_mode_convert_umode(struct drm_display_mode *out,
 		goto out;
 	}
 
-	if ((in->flags & DRM_MODE_FLAG_3D_MASK) > DRM_MODE_FLAG_3D_MAX)
-		goto out;
-
 	out->clock = in->clock;
 	out->hdisplay = in->hdisplay;
 	out->hsync_start = in->hsync_start;