drm/i915: check port power domain when reading the encoder hw state

Since the encoder is tied to its port, we need to make sure the power
domain for that port is on before reading out the encoder HW state.

Note that this also covers also all connector get_hw_state handlers,
since all those just call the corresponding encoder get_hw_state
handler, which checks - after this change - for all power domains
the connector needs.

v2:
- no change
v3:
- push down the power domain checks into the specific encoder
  get_hw_state handlers (Daniel)

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>