drm/i915: Warn if stealing power sequencer from an active eDP port

eDP ports need the power seqeuncer whenever the port is active. Warn if
we accidentally steal the power sequener from an active eDP port. This
should not happen unless there's a bug somewhere else, but it's best to
scream loudly if it happens to help with debugging.

Note that this only checks for active pipes and not for enabled pipes
which are turned off with dpms. Which means we might run the risk that
the pps might get stolen and we can't reacquire one when enabling the
pipe again with dpms on. But on current platforms that's impossible
since we only support two edp ports with just two panel power
sequencers. So a more elaborate scheme which reserves the pps even
when the pipe is inactive isn't required.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
[danvet: Summarize my discussion with Ville about dpms on/off issues.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>