drm: Skip drm_mode_config_validate() for !modeset

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: 74d2aacbe840 ("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.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>