drm/amd/display: Take full update path if number of planes changed

[Description]
- A full update is required if the number of planes for a given
  stream changes
- The new fast update path only checked for stream and plane updates,
  but there could be a plane addition or removal without one of the
  stream and plane updates triggering a full update
- Add an explicit check for number of planes changing for a full update

Reviewed-by: Samson Tam <samson.tam@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>