drm/omap: refactor CRTC HW property setup

The current driver doesn't expose any of the CRTC HW properties like
background color or transparency key, and sets them at CRTC enable time.

Refactor this into a separate function and call that function from
omap_crtc_atomic_flush(). This is the behavior we want when the
properties can be configured, so this patch makes it easier to add
patches later which implement those properties.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>