[Why]
The DRM overlay planes DM exposes support RGBA formats but are currently
forced as fully opaque over whatever they overlay.

[How]
Expose DRM blending mode and alpha properties to userspace.

The overlays exposed support per-pixel pre-multiplied alpha along with
global plane opacity.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>