drm/msm/atomic: Switch to vblank_start helper

Drop our custom thing and switch to drm_crtc_next_vblank_start() for
calculating the time of the start of the next vblank period.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/525819/
Link: https://lore.kernel.org/r/20230308155322.344664-15-robdclark@gmail.com