drm/amdgpu:cancel timer of virtual DCE(v2)

virtual DCE Timer structure is already released
after its sw_fini(), so we need to cancel the
its Timer in hw_fini() otherwise the Timer canceling
is missed.

v2:
use for loop and num_crtc to replace original code

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>