drm/amdgpu: increase the MAX ring number

As two more SDMA page queue rings are added on Vega20.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>