anv: Allow all clear colors for texturing on Gen11+

Starting with Gen11, we have two indirect clear colors: An unconverted
float/int version which is us used for rendering and a converted pixel
value version which is used for texturing.  Because the one used for
texturing is stored as a single pixel of that color, it works no matter
what format is being used.  Because it's a simple HW indirect and
doesn't involve copying surface states around, we can use it in the
sampler without having to worry about surface states having out-of-date
clear values.  The result is that we can now allow any clear color when
texturing.

This cuts the number of resolves in a RenderDoc trace of Dota2 by 95%
on Gen11+ (you read that right) and improves perf by 3.5%.  It improves
perf in a handful of other workloads by < 1%.

Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Part-of: <mesa/mesa!4393>
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