zink: fix 8 bit index handling code

index_size is specified in bytes, not bits.

Fixes: f4583b40863 ("zink: move 8bit index handling out of u_primconvert path")

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8081>
(cherry picked from commit ba74e1be22f646f9639e85b12c7707e96351a075)

Conflicts:
	src/gallium/drivers/zink/zink_draw.c
2 files changed