msm: kgsl: Check for SMMU v2 version only once

Check for SMMU v2 version only once and cache the result,
making this check every time is expensive operation and can
affect performance.

CRs-fixed: 641951
Change-Id: Ica57561d3910e3144bd31543852a9d005273a1d0
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>