drm/i915: use power get/put instead of set for power on after init

Currently we make sure that all power domains are enabled during driver
init and turn off unneded ones only after the first modeset. Similarly
during suspend we enable all power domains, which will remain on through
the following resume until the first modeset.

This logic is supported by intel_set_power_well() in the power domain
framework. It would be nice to simplify the API, so that we only have
get/put functions and make it more explicit on the higher level how this
"power well on during init" logic works. This will make it also easier
if in the future we want to shorten the time the power wells are on.

For this add a new device private flag tracking whether we have the
power wells on because of init/suspend and use only
intel_display_power_get()/put(). As nothing else uses
intel_set_power_well() we can remove it.

This also fixes

commit 6efdf354
Author: Imre Deak <imre.deak@intel.com>
Date:   Wed Oct 16 17:25:52 2013 +0300

    drm/i915: enable only the needed power domains during modeset

where removing intel_set_power_well() resulted in not releasing the
reference on the power well that was taken during init and thus leaving
the power well on all the time. Regression reported by Paulo.

v2:
- move the init_power_on flag to the power_domains struct (Daniel)

v3:
- add note about this being a regression fix too (Paulo)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>