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>