softpipe: fix texture view crashes

I noticed we crashed piglit arb_texture_view-rendering-formats
when run on softpipe.

This fixes the clear tiles to use the surface format not the
underlying storage format.

This fixes a bunch of srgb piglits as well.

Fixes: 396ac41f (softpipe: add integer support)

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
(cherry picked from commit 04189565)