drm/omap: check that plane is inside crtc

DRM allows planes to be partially off-screen, but DSS hardware does not.
This patch adds the necessary check to reject plane configs if the plane
is not fully inside the crtc.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>