drm/amdgpu: Remove warning for virtual_display

Remove the virtual_display warning in drm_crtc_vblank_off when
dev->num_crtcs is null.

Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Emily.Deng <Emily.Deng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>