msm: mdss: fix error checking for dma_map_sg_lazy

dma_map_sg_lazy returns the nents value, ie the number of entries in
sglist which is passed on success. Error checking should be done based
on nents value and not on non-zero return value.

Change-Id: I341990fcd4eb123306fb8d1c3b08be85f0b7764b
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>