drm/amd/display: Remove Assert from dcn10_get_dig_frontend

[Why]
In some cases, this function is called when DIG BE is not
connected to DIG FE, in which case a value of zero isn't
invalid and assert should not be hit.

[How]
Remove assert and handle ENGINE_ID_UNKNOWN result in calling
function.

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>