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 <dillon.varone@amd.com>
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>