drm/i915/audio: fix check for av_enc_map overflow

Turns out -1 >= ARRAY_SIZE() is always true. Move the bounds check where
we know pipe >= 0 and next to the array indexing where it makes most
sense.

Fixes: 9965db26ac05 ("drm/i915: Check for fused or unused pipes")
Fixes: 0b7029b7e43f ("drm/i915: Check for fused or unused pipes")
Cc: <stable@vger.kernel.org> # v4.10+
Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180214173840.25360-1-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_audio.c

@@ -779,11 +779,11 @@ static struct intel_encoder *get_saved_enc(struct drm_i915_private *dev_priv,
 {
 	struct intel_encoder *encoder;
 
-	if (WARN_ON(pipe >= ARRAY_SIZE(dev_priv->av_enc_map)))
-		return NULL;
-
 	/* MST */
 	if (pipe >= 0) {
+		if (WARN_ON(pipe >= ARRAY_SIZE(dev_priv->av_enc_map)))
+			return NULL;
+
 		encoder = dev_priv->av_enc_map[pipe];
 		/*
 		 * when bootup, audio driver may not know it is