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

So far the modeset code enabled all power domains if it needed any. It
wasn't a problem since HW generations so far only had one always-on
power well and one dynamic power well that can be enabled/disabled. For
domains powered by always-on power wells (panel fitter on pipe A and the
eDP transcoder) we didn't do anything, for all other domains we just
enabled the single dynamic power well.

Future HW generations will change this, as they add multiple dynamic
power wells. Support for these will be added later, this patch prepares
for those by making sure we only enable the required domains.

Note that after this change on HSW we'll enable all power domains even
if it was the domain for the panel fitter on pipe A or the eDP
transcoder. This isn't a problem since the power domain framework
already checks if the domain is on an always-on power well and doesn't
do anything in this case.

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>