drm: omapdrm: merge omap_crtc_flush and omap_crtc_atomic_flush

omap_crtc_atomic_flush() is the only user of omap_crtc_flush(), so just
move the code from omap_crtc_flush() to omap_crtc_atomic_flush().

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