renderer_vulkan/fixed_pipeline_state: Move enabled bindings to static state

Without using VK_EXT_robustness2, we can't consider the 'enabled' (not
null) vertex buffers as dynamic state, as this leads to invalid Vulkan
state. Move this to static state that is always hashed and compared in
the pipeline key.

The bits for enabled vertex buffers are moved into the attribute state
bitfield. This is not 'correct' as it's not an attribute state, but that
struct has bits to spare, and it's used in an array of 32 elements (the
exact same number of vertex buffer bindings).