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>