ion: change ion kernel map function to not take flags argument

Buffer flags are going to be specified at allocation time rather than
map time. This removes the flags argument from the ion kernel map
function.

Change-Id: Ib983ecd0dcd7befb36287ae7037c71d4ca475f90
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>