llvmpipe: get rid of llvmpipe_get_texture_image_all

llvmpipe: get rid of llvmpipe_get_texture_image_all

Once used for invoking swizzled->linear conversion for all needed images.
But we now have a single allocation for all images in a resource, thus looping
through all slices is rather pointless, conversion doesn't happen neither.
Also simplify the sampling setup code to use the mip_offsets array in the
resource directly - if the (non display target) resource exists its memory
will already be allocated as well.

Reviewed-by: Brian Paul <brianp@vmware.com>