iris: Stall on the CPU and resolve predication during fast clears.

iris: Stall on the CPU and resolve predication during fast clears.

Only if the clear color/depth is changing. In those cases, it's hard to
keep track of the current clear color, and aux state of some layers,
when predication is enabled. So simplify everything by stalling on the
few cases where we would have a fast clear color change with
predication.

v2:
 - fix comment (Ken)
 - explicitly check for predicate state after resolving it (Ken)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>