drm: omapdrm: Register omapdrm platform device in omapdss driver

The omapdrm platform device is a virtual device created for the sole
purpose of handling the omapdss/omapdrm driver split. It should
eventually be removed. As a first step to ease refactoring move its
registration from platform code to driver code.

The omapdrm driver name must be changed internally to avoid probing both
the device registered in platform code and the device registered in the
omapdss driver, as that would otherwise break bisection.

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