gpu: ion: Flush mappings before making a secure call

Before making a call to protect a buffer, we need to ensure
there are no outstanding mappings. If the buffers are in
highmem, there may be mappings left over from old kmap calls.
Call the appropriate APIs to remove the extra mappings before
making a secure call.

Change-Id: I8f573fbfa2b2ca1b40ad5ca4eab79d916d3b00ee
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>