anv: Only fast clear single-slice images

anv: Only fast clear single-slice images

The current strategy we use for managing resolves has an issues where we
track clear colors and the need for resolves per-LOD but we still allow
resolves of only a subset of the slices in any given LOD and doing so
sets the "needs resolve" flag for that LOD to false while leaving the
remaining layers unresolved.  This patch is only the first step and does
not, by itself fix anything.  However, it's fairly self-contained and
splitting it out means any performance regressions should bisect to this
nice obvious commit rather than to the giant "rework aux tracking"
commit.

Nanley and I did some testing and none of the applications we tested
even tried to fast-clear anything other than the first slice of an
image.  The test was done by adding a printf right before we call
blorp_fast_clear if we were every going to touch any slice other than
the first with a fast-clear.  Due to the way the original code was
structured, this would not have included applications which only cleared
a subset of layers.  The applications tested were:

 * All Sascha Willems demos
 * Aztec Ruins
 * Dota 2
 * The Talos Principle
 * Mad Max
 * Warhammer 40,000: Dawn of War III
 * Serious Sam Fusion 2017: BFE

While not the full list of shipping applications, it's a pretty good
spread and covers most of the engines we've seen running on our driver.
If this is ever shown to be a performance problem in the future, we can
reconsider our strategy.

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>