i965: Avoid unnecessary copy when depthstencil workaround invoked by clear.

i965: Avoid unnecessary copy when depthstencil workaround invoked by clear.

Since apps typically begin rendering with a call to glClear(), it is
likely that when brw_workaround_depthstencil_alignment() moves a
miplevel to a temporary buffer, it can avoid doing a blit, since the
contents of the miplevel are about to be erased.

This patch adds the necessary plumbing to determine when
brw_workaround_depthstencil_alignment() is being called as a
consequence of glClear(), and avoids the unnecessary blit when it is
safe to do so.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
v2: Eliminate unnecessary call to _mesa_is_depthstencil_format().  Fix
handling of depth buffer in depth/stencil format.

v3: Use correct bitfields for clear_mask.  Fix handling of depth
buffer in depth/stencil format when hardware uses separate stencil.
When invalidating, make sure we still reassociate the image to the new
miptree.

Reviewed-by: Eric Anholt <eric@anholt.net>