backend-drm: Pass view alpha to plane state and commit

We map view alpha(0.0-1.0) to plane state's alpha
by using the max plane alpha value got from drm.

Signed-off-by: Hsuan-Yu Lin <hlin@jp.adit-jv.com>
Signed-off-by: Veeresh Kadasani <external.vkadasani@jp.adit-jv.com>
Signed-off-by: Vinh Nguyen Trong <Vinh.NguyenTrong@vn.bosch.com>