drm/msm/mdp4: flush vblank event on disable

Flush queued events when disabling the crtc. This avoids timeouts when
we come back and wait for dependencies (like the previous frame's
flip_done).

Fixes: c8afe684

 ("drm/msm: basic KMS driver for snapdragon")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/569127/
Link: https://lore.kernel.org/r/20231127215401.4064128-1-dmitry.baryshkov@linaro.org