gpu: ion: Only map as much of the vma as the user requested

Change-Id: I62f60cb195b1be6655a3dbb518966e6bae94cb42
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Git-commit: d1d2e536b1920ff7c2fe1a5a8967fe7075a85e71
Git-repo: https://android.googlesource.com/kernel/common


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