msm: ipa3: set GPI channels to DB mode

In order to enable prefetch control in GSI,
channels needs to be configured to doorbell mode.

CRs-Fixed: 1000819
Change-Id: I4847982f48b09de1690bb474db9a60e018e0c0d6
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>