drm/amdgpu/vg20:increase 3 rings for AMDGPU_MAX_RINGS

For Vega20, there are two UVD Hardware. One more UVD hardware
adds one decode ring and two encode rings. So AMDGPU_MAX_RINGS
need increase by 3.

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