intel/blorp: Define the clear value bounds for HiZ clears

intel/blorp: Define the clear value bounds for HiZ clears

Follow the restriction of making sure the clear value is between the min
and max values defined in CC_VIEWPORT. Avoids a simulator warning for
some piglit tests, one of them being:

./bin/depthstencil-render-miplevels 146 d=z32f_s8

Jason found this to fix incorrect clearing on SKL.

Fixes: 09948151ab1d5184b4dd9052bb1f710fa1e00a7b
       ("intel/blorp: Add the BDW+ optimized HZ_OP sequence to BLORP")

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Tested-by: Jason Ekstrand <jason@jlekstrand.net>