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: 09948151
       ("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>
(cherry picked from commit 5bcf4795)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	src/intel/blorp/blorp_genX_exec.h