drm/vkms: Assert if vkms_config_create_*() fails

drm/vkms: Assert if vkms_config_create_*() fails

Check that the value returned by the vkms_config_create_*() functions is
valid. Otherwise, assert and finish the KUnit test.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/dri-devel/aJTL6IFEBaI8gqtH@stanley.mountain/
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20250811101529.150716-1-jose.exposito89@gmail.com
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>