drm/i915: runtime PM support for DPMS

drm/i915: runtime PM support for DPMS

Keeping track of the power domains is a bit messy since crtc->active
is currently updated by the platform hooks, but we need to be aware of
which state transition exactly is going on. Maybe we simply need to
shovel all the power domain handling down into platform code to
simplify this. But doing that requires some more auditing since
currently the ->mode_set callbacks still read some random registers
(to e.g. figure out the reference clocks).

Also note that intel_crtc_update_dpms is always call first/last even
for encoders which have their own dpms functions. Hence we really only
need to update this place here.

Being a quick "does it blow up?" run not really tested yet.

v2: Don't do runtime PM in the DPMS hooks for HAS_DDI platforms since
that is stalled. Also add a comment to explain what's going on.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>