r600g: do fine-grained depth texture flushing

r600g: do fine-grained depth texture flushing

- maintain a mask of which mipmap levels are dirty (instead of one big flag)
- only flush what was requested at a given point and not the whole resource
  (most often only one level and one layer has to be flushed)

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>