drm/amd/display: Fix dynamic encoder reassignment

[Why]
Incorrect encoder assignments were being used while applying a new state
to hardware.

(1) When committing a new state to hardware requires resetting the
back-end, the encoder assignments of the current or old state should be
used when disabling the back-end; and the encoder assignments for the
next or new state should be used when re-enabling the back-end.

(2) Link training on hot plug could take over an encoder already in use
by another stream without first disabling it.

[How]

(1) Introduce a resource context 'link_enc_cfg_context' which includes:
- a mode to indicate when transitioning from current to next state.
- transient encoder assignments to use during this state transition.

Update the encoder configuration interface to respond to queries about
encoder assignment based on the mode of operation.

(2) Check if an encoder is already in use before attempting to perform
link training on hot plug.

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>