drm/amd/display: Create plane rotation property

It's otherwise properly supported, just needs exposing to userspace.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Pierre-Loup A. Griffais <pgriffais@valvesoftware.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>