ion: msm: fix bug with handle id handling

0 is not a valid handle id value. If we get a 0 we should try to do the
dma_buf import.

Change-Id: I2b9355a5ad65c91247c0369047f921cf02908b1c
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
ion: invalidate cache after zero'ing buffers even if uncached

Currently, we don't invalidate the cache if we're working with uncached
buffers. However, the memory for those mappings might actually be stale
in the cache since someone else might have touched it. Just invalidate
the cache for everyone, cached or uncached.

Without this change, corruption was observed on the display due to stale
data in graphics buffers from Ion.

CRs-Fixed: 585524
Change-Id: I0037beaadfde983af15d8f45918f7798264dd3ca
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
ion: remove unused function

Somehow, during [83c50939e88d0 gpu: ion: optimize zero'ing of pages] we
retained some cruft. Remove it.

Change-Id: I38f2ef5d6cd1baa3fe8a758a0bf8164695e2b657
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>