drm/msm/sde: fix pending kickoff refcount for dual intf

Current driver increments the pending kickoff
counter only one time per virtual encoder.
This is a problem when we have one virtual
encoder and two physical encoders since
driver will not wait for both pp done
interrupts to be signaled before preparing
the next commit. Fix this issue by making
sure the kickoff pending counter is
incremented per physical encoder.

Change-Id: I94b20164207730ad7098ce48622c1d98bcda7038
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>