drm/i915/dp: Wait for PP_CONTROL to take effect.

This patch fixes the black screen bug on Dell e6510, by
adding two delays to give the eDP panel time to turn on before we
continue with the next write.

300ms is rather arbitray and a rather long sleep, we need to find a way
of refining this value.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=29278


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>