drm/amd/display: Fix concurrent dynamic encoder assignment

[Why]
Trying to enable multiple displays simultaneously exposed shortcomings
with the algorithm for dynamic link encoder assignment.

The main problems were:
- Assuming stream order remained constant across states would sometimes
lead to invalid DIG encoder assignment.
- Incorrect logic for deciding whether or not a DIG could support a
stream would also sometimes lead to invalid DIG encoder assignment.
- Changes in encoder assignment were wholesale while updating of the
pipe backend is incremental. This would lead to the hardware state not
matching the software state even with valid encoder assignments.

[How]

The following changes fix the identified problems.
- Use stream pointer rather than stream index to track streams across
states.
- Fix DIG compatibility check by examining the link signal type rather
than the stream signal type.
- Modify assignment algorithm to make incremental updates so software
and hardware states remain coherent.

Additionally:
- Add assertions and an encoder assignment validation function
link_enc_cfg_validate() to detect potential problems with encoder
assignment closer to their root cause.
- Reduce the frequency with which the assignment algorithm is executed.
It should not be necessary for fast state validation.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>