drm: omapdrm: Don't flush CRTC when enabling or disabling it

The omap_crtc_flush() call in omap_crtc_enable() and omap_crtc_disable()
is a no-op, as the display manager is always disabled at this point. Just
remove the function call.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>