drm: omapdrm: fb: Limit number of planes per framebuffer to two

The only multi-planar format supported by the driver is NV12, there will
thus never be more than two planes per framebuffer.

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