i965: Refactor brw_is_hiz_depth_format()

i965: Refactor brw_is_hiz_depth_format()

Every caller of this function uses it to determine if the current
miptree needs a hiz buffer to be allocated. Strangely, the function
doesn't take a miptree argument. So, this function effectively decides
if and when a miptree's hiz buffer gets allocated without inspecting the
miptree itself.  Luckily, the driver behaves correctly despite the
brw_is_hiz_depth_format's quirk.

I will soon make some changes to the miptree that will require
inspecting the miptree to determine if it needs a hiz buffer. So this
patch renames
    brw_is_hiz_depth_format -> intel_miptree_wants_hiz_buffer
and gives it a miptree parameter.

This patch shouldn't change any behavior.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>