gbm: don't pass invalid arguments to gbm_surface_create_with_modifiers
gbm: don't pass invalid arguments to gbm_surface_create_with_modifiers

if num_modifiers is 0, modifiers has to be NULL.

Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

waffle-gl/waffle,waffle-gl/waffle,waffle-gl/waffle,waffle-gl/waffle