drm/nouveau: Pause between setting gpu to D3hot and cutting the power

drm/nouveau: Pause between setting gpu to D3hot and cutting the power

On the MacBook Pro, power of the gpu is cut by a gmux chip. Sometimes
the gpu gets stuck in powersaving mode and refuses to wake up
("Refused to change power state, currently in D3"). Inserting a
delay between setting the gpu to D3hot and cutting the power seems
to help (most of the time). This issue and its (partial) remediation
by the patch was observed with an Nvidia GT650M (NVE7 / GK107).

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>