drm/msm/sde: reset vsync and underrun cnt only during init

Reset vysnc and underrun count only during encoder
initialization phase instead of enable/disable to avoid
reading it zero. An encoder client needs to read
the counter before using encoder and after using it to
find the underrun counter or number of update during
cycle.

Change-Id: I96eaeb10014cabb61ddebb4676b4bb54be6dfa59
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>