drm/amd/display: Fix entry into transient encoder assignment mode.

[Why]
In some scenarios it is possible for the encoder assignment module to be
set to "transient" mode even though there are no new encoder
assignments.

This can lead to incorrect results when querying encoder assignment,
which in turn can cause incorrect displays to be manipulated.

[How]
Only allow encoder assignment to be in transient mode of operation when
there are valid new encoder assignments.

Reviewed-by: Meenakshikumar Somasundaram <Meenakshikumar.Somasundaram@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@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>