radeonsi: fix UNSIGNED_BYTE index buffer fallback with non-zero start (v2)
radeonsi: fix UNSIGNED_BYTE index buffer fallback with non-zero start (v2)

start can only be non-zero with MultiDrawElements, which is unlikely
to occur with UNSIGNED_BYTE indices.

v2: Also fix the util_shorten_ubyte_elts_to_userptr call.
    Tested with the new piglit.

Cc: 59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org
Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler