intel: enable HiZ on Gen7+

The support is restricted to Gen7+.  To support HiZ on Gen6, we need to
perform manual combined/separated stencil resolves and that may hurt the
performance instead.

Also, we do not support fast depth clears.  xglSetFastClearDepth() will be
removed in a later xgl.h revision.
diff --git a/icd/intel/intel.c b/icd/intel/intel.c
index c5b4f63..2ffdad2 100644
--- a/icd/intel/intel.c
+++ b/icd/intel/intel.c
@@ -60,6 +60,8 @@
                 intel_debug |= INTEL_DEBUG_NOHW;
             } else if (strncmp(env, "nocache", len) == 0) {
                 intel_debug |= INTEL_DEBUG_NOCACHE;
+            } else if (strncmp(env, "nohiz", len) == 0) {
+                intel_debug |= INTEL_DEBUG_NOHIZ;
             } else if (strncmp(env, "hang", len) == 0) {
                 intel_debug |= INTEL_DEBUG_HANG;
             } else if (strncmp(env, "0x", 2) == 0) {