msm: gsi: add new API to check (GPI) channel emptiness

This is needed by clients to determine idleness of a channel
before operations like disconnect, going to low power mode, etc.

Change-Id: I6d84e92d20a0a977e578ac366f1b1aaf4d2937b9
Signed-off-by: Ravi Gummadidala <rgummadi@codeaurora.org>