drm/i915: WARN in case we're enabling the pipe and it's enabled

... and QUIRK_PIPEA_FORCE is not present.

I initially thought that case was impossible and just added a WARN on
it, but then I was told this case is possible due to
QUIRK_PIPEA_FORCE. So let's add a WARN that serves two purposes:
  - tell us in case we have done something wrong;
  - document the only case where we expect this.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -1793,8 +1793,11 @@ static void intel_enable_pipe(struct intel_crtc *crtc)
 
 	reg = PIPECONF(cpu_transcoder);
 	val = I915_READ(reg);
-	if (val & PIPECONF_ENABLE)
+	if (val & PIPECONF_ENABLE) {
+		WARN_ON(!(pipe == PIPE_A &&
+			  dev_priv->quirks & QUIRK_PIPEA_FORCE));
 		return;
+	}
 
 	I915_WRITE(reg, val | PIPECONF_ENABLE);
 	POSTING_READ(reg);