i965/gen7+: Disentangle MSAA layout from fast clear state.

i965/gen7+: Disentangle MSAA layout from fast clear state.

This patch renames the enum that's used to keep track of fast clear
state from "mcs_state" to "fast_clear_state", and it removes the enum
value INTEL_MCS_STATE_MSAA (which previously meant, "this is an MSAA
buffer, so we're not keeping track of fast clear state").  The only
real purpose that enum value was serving was to prevent us from trying
to do fast clear resolves on MSAA buffers, and it's just as easy to
prevent that by checking the buffer's msaa_layout.

This paves the way for implementing fast clears of MSAA buffers.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>