gpu: ion: Switch to using kmalloc rather than kmap during allocation

Previously, metadata was stored in the allocated pages themselves
during allocation.  However the system can only have a limited
number of kmapped pages.  A very large allocation might exceed
this limit.

Change-Id: Ibe972096e83924bf5e621d8282c4cd133ca75b0a
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>