Documentation/drm: Update rotation property

Moving rotation property to "Drm" and removing from i915 and omap.
Also, adding description to the property

Cc: DRI Development <dri-devel@lists.freedesktop.org>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>