drm: Add rotation value to plane state

The rotation property is shared by multiple drivers, so it makes sense
to store the rotation value (for atomic-converted drivers) in the common
plane state so that core code can eventually access it as well.

Cc: dri-devel@lists.freedesktop.org
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>