gallium: clean-up, simplification of mipmapped textures

gallium: clean-up, simplification of mipmapped textures

Remove pipe_texture->first_level (always implicitly zero).  This means there's
never any unused mipmap levels at the top.
In the state tracker, we no longer re-layout mipmapped textures if the
MinLod/MaxLod texture parameters change.  It's up to the driver to obey the
pipe_sampler->min/max_lod clamps.