i965/tiled_memcpy: Rework the RGBA -> BGRA mem_copy functions

i965/tiled_memcpy: Rework the RGBA -> BGRA mem_copy functions

This splits the two copy functions into three: One for unaligned copies,
one for aligned sources, and one for aligned destinations.  Thanks to the
previous commit, we are now guaranteed that the aligned ones will *only*
operate on aligned memory so they should be safe.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93962
Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>