drm: Skip __drm_mode_set_config_internal() on atomic drivers

Everything (apart from the actual ->set_config() call)
__drm_mode_set_config_internal() does is now useless on
atomic drivers. So let's just skip all the foreplay.

v2: Use drm_drv_uses_atomic_modeset() (Daniel)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180705190010.19836-1-ville.syrjala@linux.intel.com