msm: kgsl: Do not allow uncached buffers to be marked iocoherent

Add a check to allow only cached buffers to be marked iocoherent.
We can't enable io-coherency for uncached buffers because of
situations where hardware might snoop the cpu caches which can
have stale data. This happens primarily due to the limitations
of dma caching APIs available on arm64.

Change-Id: Iafad086d38559139cb45f6a290071ec6d58ad3a3
Signed-off-by: Urvashi Agrawal <urvaagra@codeaurora.org>
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>