drm/amdgpu: Refine function name

there is no functional changes.just
refine function name to keep
consistence with other files.

change amdgpu_get_sdma_instance to
amdgpu_sdma_get_instance_from_ring.
suggested by alex.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Flora Cui <flora.cui@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>