iris: Fix blits with S8_UINT destination

For depth and stencil blits, we always want the main mask to be Z, and
the secondary pass mask to be S.  If asked to blit Z+S to S, we should
handle the blit in the second pass which properly gets the stencil
resources.

Before, we were trying to handle S as the main mask, and accidentally
blitting a Z source to a S destination, which doesn't work out well.

Fixes Piglit's "framebuffer-blit-levels {draw,read} stencil" tests.
15 jobs for !486 with st-stencil-blits in 30 minutes and 30 seconds (queued for 4 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #181823
ubuntu

00:01:24

 
  Build+Test
passed #181834
autotools

00:21:00

passed #181827
meson-clang

00:08:33

passed #181831
meson-gallium-clover-llvm

00:05:12

passed #181832
meson-gallium-clover-llvm39

00:04:46

passed #181830
meson-gallium-drivers-other

00:08:55

passed #181829
meson-gallium-radeonsi

00:06:14

passed #181833
meson-gallium-st-other

00:07:07

passed #181828
meson-gallium-swr

00:10:11

passed #181826
meson-glvnd

00:06:29

passed #181825
meson-loader-classic-dri

00:06:55

passed #181824
meson-vulkan

00:02:47

passed #181836
scons-llvm

00:06:10

passed #181835
scons-nollvm

00:05:25

passed #181837
scons-swr

00:06:20