softpipe: fix using non-zero layer in non-array view from array resource

softpipe: fix using non-zero layer in non-array view from array resource

For vertex/geometry shader sampling, this is the same as for llvmpipe - just
use the original resource target.
For fragment shader sampling though (which does not use first-layer based mip
offsets) adjust the sampling code to use first_layer in the non-array cases.
While here also fix up some code which looked wrong wrt buffer texel fetch
(no piglit change).

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>