intel: Drop texture border support code.

intel: Drop texture border support code.

Now that texture borders are gone, we never need to allocate our
textures through non-miptrees, which simplifies some irritating paths.

v2: Remove the !mt support case from intel_map_texture_image()

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
Reviewed-by: Brian Paul <brianp@vmware.com>