drm: Adding drm helper function drm_plane_from_index().

Adding drm helper function to return plane pointer from index where
index is a returned by drm_plane_index.

v2:
-avoided nested loop by adding loop count (Daniel)

v3:
-updated patch header prefix to 'drm' (Matt)

v4:
-fixed a kerneldoc issue (kbuild-internal)

Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Chandra Konduru <chandra.konduru@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>