drm/amd/display: flatten aux_engine and engine

[Why]
engine and aux_engine are unnecessary layers we want to remove this
layer.

[How]
flatten engine and aux engine structs into one struct called
aux_engine and remove all references to the engine struct.

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>