drm/amd/display: Engine-specific encoder allocation

[WHY]
From DCE110 onward, we have the ability to assign DIG BE and FE
separately for any display connector type; before, we could only do this
for DP.

Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>