drm/st7586: Remove sending of vblank event

The atomic helpers automatically send out fake VBLANK events if no
vblanking has been initialized. Remove the sending code from the
driver.

v4:
	* separate commit from core vblank changes

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200129120531.6891-12-tzimmermann@suse.de