be more clever about different framebuffer state with/without zs

In debugging krh/mesa#16 (moved) I noticed that gallium HUD rebinds the color buffer without z/s to render the HUD overlay, which triggers an extra draw pass. For example (w/ cffdump --script parse-submits.lua)

110: Batch:
-------
  # of draws: 3
  mode: RM6_GMEM
  bin size: 352x256 (9 bins)
  DEPTHTEST DEPTHWRITE
  MRT[0x44d0000]:       1024x768                RB6_R8G8B8A8_UNORM      CLEARED RESOLVED
  MRT[0x41d0000]:       1024x768                DEPTH6_24_8     CLEARED RESOLVED

111: Batch:
-------
  # of draws: 20
  mode: RM6_GMEM
  bin size: 512x384 (4 bins)
  MRT[0x44d0000]:       1024x768                RB6_R8G8B8A8_UNORM      RESTORED        RESOLVED

112: NULL BATCH!

(we also end up w/ some null/no-op batches.. presumably because a fence was requested.. not sure if that hurts too much other than extra submit ioctls)

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