msm: kgsl: Use sg->length instead of sg_dma_len API to get sg size

The CONFIG_NEED_SG_DMA_LENGTH option is set for the arm64 kernel
config. This results in the sg_dma_len API to select sg->dma_length
for the sg len which is never set and is always 0. Therefore,
change to sg->length which is what we set and use in our code.

Change-Id: I11aabf4c56bf05ba4e1c5f582b739f3b3503ec63
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>