gallium/u_blitter: implement mipmap generation

gallium/u_blitter: implement mipmap generation

for pipe_context::generate_mipmap

first move some of the blit code from util_blitter_blit_generic
to a separate function, then use it from util_blitter_generate_mipmap

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>