drm/amdgpu: Use drm_crtc_vblank_on/off for dce6

drm/amdgpu: Use drm_crtc_vblank_on/off for dce6

This old code pattern was reintroduced in

Author: Ken Wang <[email protected]>
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 <[email protected]>
Cc: Ken Wang <[email protected]>
Cc: Alex Deucher <[email protected]>
Acked-by: Christian König <[email protected]> for the whole series.
Reviewed-by: Alex Deucher <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]