drm/radeon/pm: adjust display configuration after powerstate

set_power_state defaults to no displays, so we need to update
the display configuration after setting up the powerstate on the
first call. In most cases this is not an issue since ends up
getting called multiple times at any given modeset and the proper
order is achieved in the display changed handling at the top of
the function.

Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Jordan Lazare <Jordan.Lazare@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org