msm: mdss: use appropriate api to get length of scatterlist

Moving away from using sg_dma_len() api to be inline with
ion driver, use sg->length for scatterlist length.

Change-Id: I3a33340dc1bda5b24a406c29601aff4d5b2c5748
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>