i965: Implement fast color clears using meta operations

i965: Implement fast color clears using meta operations

This patch uses the infrastructure put in place by previous patches
to implement fast color clears and replicated color clears in terms of
meta operations.

This works all the way back to gen7 where fast clear was introduced and
adds support for fast clear on gen8.  It replaces the blorp path
completely and improves on a few cases.  Layered clears are now done
using instanced rendering and multiple render-target clears use a
MRT shader with rep16 writes.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>