drm: Complete moving rotation property to core

Commit 1da30627 "drm: Add rotation value to
plane state" moved the rotation property to DRM core but only did the set
property part. This does the get property part as well.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>