softpipe/buffer: load only as many components as the the buffer resource type provides

Otherwise we risk to read past the end of the buffer.

In addition, change the loop counters to unsigned to be consistent
with the types.

Fixes: afa8707b
    softpipe: add SSBO/shader atomics support.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 865b9dda)