iris: Fix blits with S8_UINT destination

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.