drm/amd/display: Change from aux_engine to dce_aux

[Why]
The aux_engine struct is needlessly complex and
is defined multiple times.  It contains function pointers
that each have only one version and are called only from
inside dce_aux.

[How]
Replace aux_engine with a new struct called dce_aux.
Remove all function pointers and call functions directly.
Remove unused functions

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>