drm/msm/sde: wait for vsync before rsc amc mode

When driver switch the rsc state from clock
mode to amc mode, a race condition can happen
where driver doesn't wait for the vsync before
the switch to amc mode happens.
This change, adds a refcount to make sure rsc
waits for the vsync before the mode switch.

Change-Id: I8db60afebc8bf91d56a1b71e6643d0db70230a5c
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>