drm: disable encoder rather than dpms off in drm_crtc_prepare_encoders()

Original behaviour will be preserved for drivers that don't implement
disable() hooks for an encoder.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>