mesa: Convert _mesa_generate_mipmap to MapTexImage()-based access.

mesa: Convert _mesa_generate_mipmap to MapTexImage()-based access.

Now that we can zero-copy generate the mipmaps into brand new
glTexImage()-generated storage using MapTextureImage(), we no longer
need to allocate image->Data in mipmap generate.  This requires
deleting the drivers' old overrides of the miptree tracking after
calling _mesa_generate_mipmap at the same time, or the drivers
promptly lose our newly-generated data.

Reviewed-by: Eric Anholt <eric@anholt.net>