drm/amdgpu/vcn: fix vcn ring test failure in igt reload test

[why]
On Renoir, vcn ring test failed on the second time insmod in the reload
test. After investigation, it proves that vcn only can disable dpg under
dpg unpause mode (dpg unpause mode is default for dec only, dpg pause
mode is for dec/enc).

[how]
unpause dpg in dpg stopping procedure.

Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Tianci Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>