msm_fb: display: change ion memory map for mdp mixer2 writeback

One way that mdp mixer2 gets writeback address is to get ion memory fd
from userspace, then mdp driver maps this fd in kernel. When mdp iommu
domain was not enabled, this maps to a physical address. Now after mdp
iommu domain is enabled, the map is changed to an iommu virtual addr.

Change-Id: I461a00cc93169de586565834af4fb9a43949cf95
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>