drm: Skip drm_mode_config_validate() for !modeset

drm_mode_config_init() may not have been called when the driver/device
doesn't support modeset. That will cause drm_mode_config_validate()
to oops. Skip the validation for !modeset.

TODO: We may want to consider calling drm_mode_config_init()
unconditionally to avoid similar issues elsewhere...

Fixes: 74d2aacb ("drm: Validate encoder->possible_clones")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200318182518.31618-1-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/drm_mode_config.c

@@ -608,6 +608,9 @@ void drm_mode_config_validate(struct drm_device *dev)
 {
 	struct drm_encoder *encoder;
 
+	if (!drm_core_check_feature(dev, DRIVER_MODESET))
+		return;
+
 	drm_for_each_encoder(encoder, dev)
 		fixup_encoder_possible_clones(encoder);