drm: Describe the Rotation property bits.

Adds clarification of the rotation property bits. I.e. rotation is
counter clockwise and that reflects are applied before any rotation.

v2: Refer to the define names instead of the property values.

Signed-off-by: Robert Fekete <robert.fekete@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>