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: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180523145718.22932-1-ville.syrjala@linux.intel.comReviewed-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -1202,7 +1202,7 @@ void assert_panel_unlocked(struct drm_i915_private *dev_priv, enum pipe pipe)
 {
 	i915_reg_t pp_reg;
 	u32 val;
-	enum pipe panel_pipe = PIPE_A;
+	enum pipe panel_pipe = INVALID_PIPE;
 	bool locked = true;
 
 	if (WARN_ON(HAS_DDI(dev_priv)))