util/blitter: iterate samples in stencil_fallback
What does this MR do and why?
util/blitter: iterate samples in stencil_fallback
this matches handling in do_blits and fixes multisampled stencil blits
fixes (nv):
GTF-GL45.gtf44.GL31Tests.texture_stencil8.texture_stencil8_gl44
cc: mesa-stable