drm/i915: cache hw power well enabled state

Jesse noticed that the punit communication needed to query the VLV power
well status can cause substantial delays. Since we can query the state
frequently, for example during I2C transfers, maintain a cached version
of the HW state to get rid of this delay.

This fixes at least one reported regression where boot time increased by
~4 seconds due to frequent power well state queries on VLV during eDP
EDID read.

This regression has been introduced in

commit bb4932c4


Author: Imre Deak <imre.deak@intel.com>
Date:   Mon Apr 14 20:24:33 2014 +0300

    drm/i915: vlv: check port power domain instead of only D0 for eDP VDD on

Reported-by: Jesse Barnes <jesse.barnes@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>