drm/i915: Warn if panel power is already on when enabling it

We should never enable the panel power twice. That would indicate a bug
somewhere else as we would need to enable the port twice without
disabling it in between. Also print the port name.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>