drm/msm: Remove redundant parameter in msm_iommu

Parameter struct sg_table *sgt in msm_iommmu_map/unmapa already
have information of buffer length, we needn't pass "len" individually.

Change-Id: I0213f00df6b16a72aeb7201378ccdf01824d891c
Signed-off-by: Kasin Li <donglil@codeaurora.org>
Signed-off-by: Yajun Li <yajunl@codeaurora.org>