{maxwell_3d,buffer_cache}: Implement memory barriers using 3D registers

Drop MemoryBarrier from the buffer cache and use Maxwell3D's register
WaitForIdle.

To implement this on OpenGL we just call glMemoryBarrier with the
necessary bits.

Vulkan lacks this synchronization primitive, so we set an event and
immediately wait for it. This is not a pretty solution, but it's what
Vulkan can do without submitting the current command buffer to the queue
(which ends up being more expensive on the CPU).