gpu: ion: Move msm specific functions to msm_ion.h

msm only functionality that google does not want belongs
in msm_ion.h, not generic ion code. Move the definitons
to msm_ion.h.

Change-Id: I16cb780dd880fd102f59a6528671c49eb374de01
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>