Move more logic into brw_emit_depthbuffer().

brw_emit_depth_stencil_hiz and gen7_emit_depth_stencil_hiz no longer
read any context state out of the "brw" structure.  This makes it safe
for these functions to be used for blorp.  Accordingly, the first
argument to these functions is now "intel" rather than "brw".
i965/blorp: Merge into brw_depthbuffer and gen7_depthbuffer.