drm/amd/display: Minimize wait for pending updates

drm/amd/display: Minimize wait for pending updates

[Why/How]
Move the wait for pending updates past prepare_bandwidth if the previous
update was not a full update to reduce the average time it takes to
complete a full update.

Reviewed-by: Dillon Varone <[email protected]>
Reviewed-by: Alvin Lee <[email protected]>
Signed-off-by: Ilya Bakoulin <[email protected]>
Signed-off-by: Zaeem Mohamed <[email protected]>
Tested-by: Daniel Wheeler <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>