nvk: drop const for descriptor set
I introduced a warning with the change to use common code. The issue is that in the common struct vk_pipeline_layout
the set_layouts
are const while nvk functions expect to get a non-const.
One easy way to fix it is to drop the const in the common struct. I do this here. The other way is to change all the places in nvk to use const. I will be happy to make that change, but wanted to ask first.