msm: sde: prevent illegal register access

If event register is called from LA VM when the HW is owned by trusted
VM, it will lead to a crash. This change adds checks to prevent illegal
register access when the HW is not owned by LA VM.

Change-Id: Ib4d956fdce4b5aae41e765d7355082c5e233efe2
Signed-off-by: Ping Li <pingli@codeaurora.org>