drm/i915/psr: Order DP aux transactions correctly

On bdw/hsw we have a separate psr dp aux registers to set up, but on
bdw it's shared with the main dp aux thing. Which means any subsequent
dp aux transaction will trample over it, and hence must be done
beforehand.

Also this means we can't do any dp aux transactions while PSR is
active, or at least we must restore the old state.

Probably need a psr disable/enable pair around dp aux transactions in
general.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Sonika Jindal <sonika.jindal@intel.com>
Cc: Durgadoss R <durgadoss.r@intel.com>
Cc: "Pandiyan, Dhinakaran" <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463590036-17824-5-git-send-email-daniel.vetter@ffwll.ch