drm/i915: Don't kick the power seqeuncer just to check if we have vdd/panel power

If there's no power sequencer assigned to the port currently we can't
very well have vdd or panel power enabled either. If we would try to
check that from the pps registers we'd need to pick a power seqeuncer
and kick it. So let's skip the register read and the kick.

Note that there's still a bit an issue about correctly recovering pps
state from resume if the bios is nasty: With this check we'll always
assume that the pps is off. But that's better done in a follow-up
patch and it shouldn't be too harmful - at most we waste time enabling
the pps if it's on already.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
[danvet: Add note about resume issues Imre spotted.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>