drm/omap: Add 'alpha' and 'pixel blend mode' plane properties

Add the following properties for planes:
* alpha
* pixel blend mode. Only "Pre-multiplied" and "Coverage" are supported

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190711135219.23402-1-jjhiblot@ti.com