drm/msm: Use drm_mode_vrefresh instead of mode->vrefresh

Use the drm_mode_vrefresh helper where we need refresh rate in case
vrefresh is empty.

Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190128204306.95076-1-sean@poorly.run



Signed-off-by: Rob Clark <robdclark@chromium.org>