msm: ipa3: Fix null pointer dereference error

If smmu is not enabled, smmu context device check
was missing before accessing it which was resulting
in null pointer dereference. Now adding check
before accessing it.

Change-Id: I94393524c79847a165ba605d34906525640b359a
Signed-off-by: Cheng Zeng <chenzeng@codeaurora.org>