softpipe: Add an extra code path for the buffer texel lookup

softpipe: Add an extra code path for the buffer texel lookup

With buffers the addressing is done on a per-byte bases so the code
path for normal textures doesn't work properly. Also add an assert
to make sure that the bit cound for storing the X coordinate is
large enough.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>