i965: Clarify check for which cache to use on Gen6 data port reads.

i965: Clarify check for which cache to use on Gen6 data port reads.

Currently, we use the Render Cache for scratch access (read/write data)
and the Sampler Cache for all read only data (pull constants).

Reversing the condition here is clearer: if the caller requested the
Render Cache, use that.  Otherwise, they requested the Data Cache
(which does not exist on Gen6) or Sampler Cache, so use the Sampler
Cache.

This should not change behavior in any way.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>