msm: sde: fix calculating the mapped buffer size

This change calculates the total size of the mapped buffer by
traversing the scatter gather list and adds the size of each
element to get the size. This change is required with the new
version of iommu driver.

Change-Id: I19982615908a1cc85162cd5cc0a1af66f8aae106
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>