drm/i915: Initialize panel_pipe to INVALID_PIPE

drm/i915: Initialize panel_pipe to INVALID_PIPE

We can always figure out which pipe is affected by the panel power
sequencer lockout mechanism. So no need for the pipe A fallback
anymore. The only case we may have to worry about is an invalid
port select in the power sequencer, but INVALID_PIPE is just fine
in that case. We'll get the WARN about the bogus pps port select
anyway.

Cc: Jani Nikula <jani.nikula@intel.com>
Suggested-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180523145718.22932-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>