msm: kgsl: Use sg table instead of physaddr to check secure guard page

Earlier, we would use the memdesc physaddr to determine whether
to a guard page for secure buffers had already been allocated.
Change that check to use the sg table since we do not store the
physaddr for memdescs which are not allocated via
dma_alloc_coherent.

Change-Id: I24a7ec2f60fbe7f43df1342d316b7bae4d86e787
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>