drm/amdgpu: remove unused variable ring

building with gcc and W=1 reports
drivers/gpu/drm/amd/amdgpu/vcn_v4_0.c:81:29: error: variable
  ‘ring’ set but not used [-Werror=unused-but-set-variable]
   81 |         struct amdgpu_ring *ring;
      |                             ^~~~

ring is not used so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>