drm/msm/sde: Add refcount support for wb irq ctrl

Writeback timeout is seen during writeback disable. This is
because IRQ registration is now moved out of prepare kickoff
and is only enabled by resource control during kickoff. So,
refcount is added for wb irq registration balancing. Also,
add wb ctrl for null commit during writeback disable.

Change-Id: I5b1b2abbf275824158a9732b89c1f35deaf347b7
Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>