zink: fix stencil-only blitting with stencil fallback
What does this MR do and why?
zink: fix stencil-only blitting with stencil fallback
these shouldn't do extra depth blits
fixes (nvidia):
dEQP-GLES3.functional.fbo.blit.depth_stencil.depth24_stencil8_stencil_only,Fail
dEQP-GLES3.functional.fbo.blit.depth_stencil.depth32f_stencil8_stencil_only,Fail
spec@arb_framebuffer_object@arb_framebuffer_object-depth-stencil-blit stencil gl_depth24_stencil8,Fail
cc: mesa-stable