drm/tiny/gm12u320: Retrieve USB device from struct drm_device.dev

Drop the driver's udev field in favor of struct drm_device.dev. No
functional changes made.

v3:
	* upcast dev with gm12u320_to_usb_device()
v2:
	* upcast dev with drm_dev_get_usb_device()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201103113456.3066-2-tzimmermann@suse.de