vhost-user-gpu: fix vugbm_device_init fallback

vhost-user-gpu: fix vugbm_device_init fallback

vugbm implements GBM device wrapping, udmabuf and memory fallback.
However, the fallback/detection logic is flawed, as if "/dev/udmabuf"
failed to be opened, it will not initialize vugbm and crash later.

Rework the vugbm_device_init() logic to initialize correctly in all
cases.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210312100108.2706195-4-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>