drm: Fix memory leak in drm_mode_setcrtc()

The mode passed to the .set_config() hook was never freed. The drivers
will make a copy of the mode, so simply free it when done.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>