drm/imx: enable/disable PRG on CRTC enable/disable

On i.MX6 QuadPlus the PRG needs to be clocked in order to pass
through the data access requests from the IDMAC. This call is a
no-op for other all other SoCs.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>