r600g: add a depth misc state which depends on occlusion queries

r600g: add a depth misc state which depends on occlusion queries

This is a state which is derived from other states and is actually the first
state which doesn't correspond to any gallium state.

There are two state flags:
  bool occlusion_query_enabled
  bool flush_depthstencil_enabled

Additional flags can be added later if needed, e.g. bool hiz_enabled.
The emit function will have to figure out the register values by itself.

It basically just emits the registers when the state changes.
This commit also adds a few helper functions for writing registers directly
into a command stream.