drm/radeon/dce4+: don't use radeon_crtc for vblank callback

commit 4a15903db02026728d0cf2755c6fabae16b8db6a upstream.

This might be called before we've allocated the radeon_crtcs

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>