drm/amdgpu: use max_dw in ring_init

Instead of specifying the total ring size calculate that from the maximum
number of dw a submission can have and the number of concurrent submissions.

This fixes UVD with 8 concurrent submissions or more.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>