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: mesa-stable@lists.freedesktop.org
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
(cherry picked from commit a264fee6245856340fab9024e1a428626e966335)
[Emil Velikov: resolve trivial conflicts]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

Conflicts:
	src/gallium/drivers/radeonsi/si_state_draw.c
1 file changed