intel/blorp: Use wide formats for nicely aligned stencil clears

intel/blorp: Use wide formats for nicely aligned stencil clears

In the case where the stencil clear is nicely aligned, we can clear
stencil much more efficiently by mapping it as a wide format (say
RGBA32_UINT) and blasting out the stencil clear value with a repclear.
On Unigine Heaven, this makes one stencil clear go from non-trivial to
unnoticeable when looking at per-draw timings.

In order for this change to work properly, ANV needs to do a bit more
flushing around depth and stencil clears.  i965 and iris already have
the cache tracking logic to handle this so no changes are required
there.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>