msm: kgsl: Fix size for device memstore allocation

Change prevents IOMMU pagefaults which may occur when
indexing reaches 4096/sizeof(kgsl_devmemstore) number
of contexts

CRs-Fixed: 370594
Change-Id: If726c2a1fcf7667b062e890453a71e95f5417c60
Signed-off-by: Richard Ruigrok <rruigrok@codeaurora.org>