gpu: ion: Switch max num_heaps to 16 to work around an issue elsewere

Change-Id: I3bcbc9c424086f5921a1641afeb16040deffcece
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
[mitchelh@codeaurora.org: applied to renamed file since rename detection
was broken since we never removed include/linux/ion.h.]
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>