gpu: ion: move msm-specific ioctls to custom_ioctl

To avoid collisions with future ioctls that may be added to ion,
msm-specific ioctls should use the `custom_ioctl' interface provided
by struct ion_device, as originally intended.

Change-Id: Ic36f1ff37ce79926482942f70d466acf95c9c170
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>