msm: gsi: reduce register accesses on data path

Optimize the way GSI driver reads writes registers in the data path.
RP (read pointer) will get read only in case the cached value cannot
be used. For event ring for FROM_GSI, the WP (write pointer, doorbell),
will be advanced based on channel credits.

Change-Id: I73267f9011c9568083e94fc5c398750318626ba5
CRs-Fixed: 2022853
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>