i965/tiled_memcopy: Get rid of the direction parameter to get_memcpy

i965/tiled_memcopy: Get rid of the direction parameter to get_memcpy

Now that we can use the much simpler rgba8_copy function, we don't need to
hand different functions out based on direction.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>