fbdev: vesafb: add dev->remove() callback

If x86-sysfb platform-devices are removed from a system, we should
properly unload vesafb. Otherwise, we end up releasing the parent while
our vesa framebuffer is still running. This currently works just fine, but
will cause problems on handover to real hw. So add the ->remove() callback
and unregister vesafb.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>