drm/msm: Change dpu_crtc_get_vblank_counter to use vsync count.

[why]
vsync_cnt atomic counter increments for every hw vsync. On the other
hand, frame count is a register that increments when the frame gets
actually pushed out. We cannnot read this register whenever the timing
engine is off, but vblank counter should still return a valid number.
This behavior also matches the downstream driver.

[How]
Read the encoder vsync count instead of the dpu_encoder_phys frame
count.

Suggested-by: Abhinav Kumar <abhinavk@codeaurora.org>
CC: Rob Clark <robdclark@chromium.org>
Signed-off-by: Mark Yacoub <markyacoub@chromium.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Link: https://lore.kernel.org/r/20210830181359.124267-1-markyacoub@chromium.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>