llvmpipe: fix bogus handling of first_layer when setting up texture sampling

llvmpipe: fix bogus handling of first_layer when setting up texture sampling

The code avoided first_layer parameter in the sampler interface (and needing
to do another calculation at runtime) by fixing up the base texture pointer
instead. Unfortunately, this didn't actually work as we have mip-first
texture layout so fixing up the base ptr by a fixed amount is very wrong if
there are mipmaps present. The wrong offsets caused misrendering and crashes.
Fix this by just adjusting the individual mip level offsets instead.
Spotted by Jose.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>