drm/amdgpu: Sync up header and implementation to use the same parameter names

Sync up header and implementation to use the same parameter names
in function amdgpu_ring_init.
ring_size -> max_dw, prio -> hw_prio

Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>