drm/i915: add port power domains

Parts that poke port specific HW blocks like the encoder HW state
readout or connector hotplug detect code need a way to check whether
required power domains are on or enable/disable these. For this purpose
add a set of power domains that refer to the port HW blocks. Get the
proper port power domains during modeset.

For now when requesting the power domain for a DDI port get it for a 4
lane configuration. This can be optimized later to request only the 2
lane power domain, when proper support is added on the VLV PHY side for
this. Atm, the PHY setup code assumes a 4 lane config in all cases.

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>