iris: Explicitly emit 3DSTATE_BTP_XS on Gen9 with DIRTY_CONSTANTS_XS

iris: Explicitly emit 3DSTATE_BTP_XS on Gen9 with DIRTY_CONSTANTS_XS

Right now, we usually flag both IRIS_DIRTY_{CONSTANTS,BINDINGS}_XS,
because we have SURFACE_STATE for constant buffers in case the shaders
access them via pull mode.

But this flagging is overkill in many cases.  Gen8 and Gen11 don't need
it at all.  Gen9 doesn't need that large of a hammer in all cases.

Just handle it explicitly so the right thing happens.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>