drm/amd/display: Initialise encoder assignment when initialising dc_state

[Why]
Link encoder assignment tracking variables need to be (re)initialised
whenever dc_state is (re)initialised. Otherwise variables used for
dynamic encoder assignment (especially the link encoder availability
pool) are out of sync with dc_state and future encoder assignments are
invalid.

[How]
Initialise encoder assignment variables when creating new dc_state
resource.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>