drm/amd/display: Maintain consistent mode of operation during encoder assignment

[Why]
While applying a state to hardware, there is a transition period where
the back-end is reset using the old state; then enabled using the new
state.

Generally, the link encoder configuration module queries
stream-to-encoder assignments in either the new or old state based on a
mode variable. During the transition there is a need to query both
states, however toggling this mode variable can lead to incorrect
programming of encoders.

[How]
- Add new function to explicity query stream-to-encoder assignment
in the current state rather than intermittently switch the mode
of operation of the link encoder assignment module.
- Add additional checks for encoder assignment defects.
- Explicitly reset the mode of operation if application of state
to hardware ends prematurely.

Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>