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>
Git-commit: b144bf48c486f6fb7a7ec5ed96af62bef586c89f
Git-repo: https://android.googlesource.com/kernel/common


[lauraa@codeaurora.org: Needed to fixup context due to parent
change being merged first in cherry-picking]
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>