intel: Replace checks for hiz_mt with intel_has*hiz()

intel: Replace checks for hiz_mt with intel_has*hiz()

When appropriate, replace each check `hiz_mt != NULL` with either a call
to intel_miptree_slice_has_hiz() or intel_renderbuffer_has_hiz().  No
behavioral change.

This prepares for selectively enabling hiz on individual miptree slices
for Haswell.

This refactoring had several side effects.

  1. To prevent new warnings about discarding the const qualifier,
     I removed 'const' from some variable declarations in
     intel_validate_framebuffer().  The alternative was to add const
     qualifiers to multiple function signatures in the
     intel_renderbuffer_has_hiz call graph. Since the dominant convention
     in the Intel code is to not qualify function parameters as const,
     I chose to remove rather than add const qualifiers.

  2. I changed the signature of brw_emit_depth_stencil_hiz() by replacing
     `struct intel_mipmap_tree *hiz_mt` with `bool hiz`. The function used
     hiz_mt mostly as a boolean indicator of the presence of hiz, so the
     signature change is consistent with the patch's goal.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>