drm/amdgpu: Use __drm_atomic_helper_crtc_reset

Now also comes with the added benefit of doing a drm_crtc_vblank_off(),
which means vblank state isn't ill-defined and fail-y at driver load
before the first modeset on each crtc.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Mikita Lipski <mikita.lipski@amd.com>
Cc: Stylon Wang <stylon.wang@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200612160056.2082681-2-daniel.vetter@ffwll.ch

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -4593,9 +4593,7 @@ static void dm_crtc_reset_state(struct drm_crtc *crtc)
 	if (WARN_ON(!state))
 		return;
 
-	crtc->state = &state->base;
-	crtc->state->crtc = crtc;
-
+	__drm_atomic_helper_crtc_reset(crtc, &state->base);
 }
 
 static struct drm_crtc_state *