msm: kgsl: Fix overflow in sharedmem read/write functions

There could be possibility of integer overflow on adding
sizeof(uint32_t) with uint64_t maximum offset bytes and result
in a value smaller than uint64_t maximum memdesc size.

CRs-Fixed: 988861
Change-Id: Ifc3ec45297c2a29ad6f7d70dd0bd59238ac8cc3d
Signed-off-by: Sunil Khatri <sunilkh@codeaurora.org>