intel: Always gather stencil buffer in intel_map_renderbuffer_separate_s8z24()

intel: Always gather stencil buffer in intel_map_renderbuffer_separate_s8z24()

The function gathered the stencil buffer into the depth buffer only when
the map mode contained the read bit. But we must do the gather even if the
map mode is write-only. If we do not, then, when the depth buffer's stencil
bits are scattered into the stencil buffer by intel_unmap_renderbuffer(),
some of the scattered stencil bits would be invalid.

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