drm/i915: fix handling gen8_emit_flush_coherentl3_wa result

The function can return negative value.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576Signed-off-by: NAndrzej Hajda <a.hajda@samsung.com>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lrc.c

@@ -1254,9 +1254,10 @@ static int gen8_init_indirectctx_bb(struct intel_engine_cs *ring,
 
 	/* WaFlushCoherentL3CacheLinesAtContextSwitch:bdw */
 	if (IS_BROADWELL(ring->dev)) {
-		index = gen8_emit_flush_coherentl3_wa(ring, batch, index);
-		if (index < 0)
-			return index;
+		int rc = gen8_emit_flush_coherentl3_wa(ring, batch, index);
+		if (rc < 0)
+			return rc;
+		index = rc;
 	}
 
 	/* WaClearSlmSpaceAtContextSwitch:bdw,chv */