msm: kgsl: Do not store timestamp for null context in pagefault handler

If the pagefault happened in a NULL context do not try to store
global timestamp information, this change prevents null pointer access
and kernel panic.

Change-Id: I42a8824adf33ad8eecceed0801146d6aefab4924
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>