drm: fix error recovery path in drm_crtc_helper_set_mode

There is no need to save or restore hwmode field, because by
the time this function sets this field, it cannot fail any more.
However, we should save old enabled field because if
the function fails, we want to return with unchanged CRTC.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>