iris: Fix SINT assert in convert_fast_clear_color

Don't assert that the size of every channel is greater than zero. This
assert doesn't work for integer formats with less than 4 channels.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7345>
1 file changed