msm: kgsl: allow creation of events from the future

For contexts created with the KGSL_CONTEXT_USER_GENERATED_TS,
allow events to be created for timestamps that have not
been issued yet. Presumably these contexts know what they
are doing.

Change-Id: Iccf2e549b38e1a11850d26ee64b7279ab0b8a1ae
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>