gpu: ion: Do the cache op when the buffer is cached

Only do the cache operation when the buffer is cached.
If the buffer is uncached, just return. Fixes a bug
where the cache was only being flushed when the buffer
was uncached instead of cached.

Change-Id: Ib1fa01168de79d5627b11d07c5313d19b3049145
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>