CHROMIUM: dma-buf/kds: allow KDS to be compiled out if dma-buf is enabled

CHROMIUM: dma-buf/kds: allow KDS to be compiled out if dma-buf is enabled

The original KDS patches added CONFIG_DMA_SHARED_BUFFER_USES_KDS but
didn't use it to guard the kds integration into dma-bufs in a way
that would allow dma-buf to be built with KDS compiled without
requiring KDS.

Fix this by more carefully guarding the integration points, and not
unconditionally selecting KDS when dma-buf is enabled.

Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=chromium:232913
TEST=compile kernel for x86 and arm and verify it still boots

Change-Id: I878d1313ce6b43477fd6c87d394728283ad4f9e6
Reviewed-on: https://gerrit.chromium.org/gerrit/48608
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>
Commit-Queue: Daniel Kurtz <djkurtz@chromium.org>