drm/amdgpu: Use drm_crtc_vblank_on/off for dce6

This old code pattern was reintroduced in

Author: Ken Wang <Qingqing.Wang@amd.com>
Date:   Tue Jan 19 14:03:24 2016 +0800

    drm/amdgpu: add display controller implementation for si v10

Realign it with later display code. Tsk, tsk for massive copypasting
;-)

Cc: Christian König <christian.koenig@amd.com>
Cc: Ken Wang <Qingqing.Wang@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com> for the whole series.
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114090255.31595-1-daniel.vetter@ffwll.ch