st/mesa: stop using gl_texture_image::Data when mapping/unmapping textures

st/mesa: stop using gl_texture_image::Data when mapping/unmapping textures

Since core Mesa no longer depends on gl_texture_image::Data pointing to
mapped texture buffers we don't have to mess with it all over the place
in the state tracker.  Now Data is only used to point to malloc'd memory
that holds images which don't fit in the texture object's mipmap buffer.