intel: Move XRGB->ARGB blit logic into intel_miptree_blit().

intel: Move XRGB->ARGB blit logic into intel_miptree_blit().

Now any caller (such as glCopyPixels()) can benefit from it, and it only
changes the correct subset of the destination instead of a whole teximage.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>