Hibernate: Add check for pte_valid in saveable page

Add check for pte_valid in saveable page after being checked for
the rest. This is required as PTE is removed for pages allocated
with __dma_alloc_coherent with DMA_ATTR_NO_KERNEL_MAPPING flag set.
This patch makes sure that these pages are not considered for
snapshot.

Change-Id: Idc7dede2d45be0a8e19899004b5277e5aba24db8
Signed-off-by: Omkar Savagaonkar <omkars@codeaurora.org>