asahi: Identify stencil test enable

asahi: Identify stencil test enable

There are a pair of flags controlling the stencil test. One enables
stencil testing in general, the other enables two-sided stencil. Compare
the identical "twosided" flag in src/imagination/csbgen/rogue_ppp.xml's
STATE_ISPCTL structure, at the samebit offset even. Evidently this word of
the "Rasterizer" is, in fact, a derivative of STATE_ISPCTL.

Fixes

   dEQP-GLES2.functional.fragment_ops.depth_stencil.*
   dEQP-GLES2.functional.fragment_ops.interaction.basic_shader.*
   dEQP-GLES2.functional.fragment_ops.random.*

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18380>