drm/tilcdc: send vblank event when disabling crtc

When run xrandr to change resolution on Beaglebone Black board, it will
print the error information:

root@beaglebone:~# xrandr -display :0 --output HDMI-1 --mode 720x400
[drm:drm_crtc_commit_wait] *ERROR* flip_done timed out
[drm:drm_atomic_helper_wait_for_dependencies] *ERROR* [CRTC:32:tilcdc crtc] commit wait timed out
[drm:drm_crtc_commit_wait] *ERROR* flip_done timed out
[drm:drm_atomic_helper_wait_for_dependencies] *ERROR* [CONNECTOR:34:HDMI-A-1] commit wait timed out
[drm:drm_crtc_commit_wait] *ERROR* flip_done timed out
[drm:drm_atomic_helper_wait_for_dependencies] *ERROR* [PLANE:31:plane-0] commit wait timed out
tilcdc 4830e000.lcdc: already pending page flip!

This is because there is operation sequence as below:

drm_atomic_connector_commit_dpms(mode is DRM_MODE_DPMS_OFF):
    ...
    drm_atomic_helper_setup_commit <- init_completion(commit_A->flip_done)
    drm_atomic_helper_commit_tail
        tilcdc_crtc_atomic_disable
        tilcdc_plane_atomic_update <- drm_crtc_send_vblank_event in tilcdc_crtc_irq
                                      is skipped since tilcdc_crtc->enabled is 0
        tilcdc_crtc_atomic_flush   <- drm_crtc_send_vblank_event is skipped since
                                      crtc->state->event is set to be NULL in
                                      tilcdc_plane_atomic_update
drm_mode_setcrtc:
    ...
    drm_atomic_helper_setup_commit <- init_completion(commit_B->flip_done)
    drm_atomic_helper_wait_for_dependencies
        drm_crtc_commit_wait   <- wait for commit_A->flip_done completing

Just as shown above, the steps which could complete commit_A->flip_done
are all skipped and commit_A->flip_done will never be completed. This will
result a time-out ERROR when drm_crtc_commit_wait check the commit_A->flip_done.
So add drm_crtc_send_vblank_event in tilcdc_crtc_atomic_disable to
complete commit_A->flip_done.

Fixes: cb345dec ("drm/tilcdc: Use standard drm_atomic_helper_commit")
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Reviewed-by: Jyri Sarha <jyri.sarha@iki.fi>
Tested-by: Jyri Sarha <jyri.sarha@iki.fi>
Signed-off-by: Jyri Sarha <jyri.sarha@iki.fi>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209082415.382602-1-quanyang.wang@windriver.com