drm/i915: vlv: increase timeout when forcing on the GFX clock

I've seen latencies up to 15msec, so increase the timeout to 20msec.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>