drm/msm/sde: manage sde pm_qos vote through atomic counter

SDE driver registers for single pm_qos node but uses it for
multiple command displays. That may lead to warnings if multiple
independent command mode displays try to keep vote at same time.
This change adds atomic refcount to add/remove the vote.

Change-Id: Idc5655bff2fea4a9724a6b9024e85ced46a9c4bc
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>