drm: Store clipped src/dst coordinatee in drm_plane_state

Pretty much all driver will have need for the clipped plane
coordinates, so let's stuff then into drm_plane_state.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1469549224-1860-3-git-send-email-ville.syrjala@linux.intel.com