backend-drm: Add plane alpha DRM-property

This checks whether plane alpha is supported.
We get range of alpha value supported for plane
which is required for mapping view's alpha(0.0-1.0)
with drm plane alpha. No functional change.

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>