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.
!UPSTREAM: Hacky fix for Gen5 depthstencil-render-miplevels stencil-only test.  TODO: think through, test thoroughly, and document.