soc: qcom: hgsl: decouple ifence and doorbell

In systems doorbell is not valid, could set db-off flag to 1. Then
the driver still could probe and disable doorbell and hfence commands.
ifence still works.

Change-Id: I6d4e171d64dfc183f5d500fbd1180adb4b108a63
Signed-off-by: Kasin Li <donglil@codeaurora.org>