msm: fb: aggregate buffer length for each sg node

when mapping the MDP framebuffer, driver needs to aggregate all
buffer length from each sg node.

CRs-Fixed: 996243
Change-Id: Ia4825e96b2e12f2328c9cc3197051f5e47c4daff
Signed-off-by: Camus Wong <camusw@codeaurora.org>