libhwcomposer: Only 4 pipes can be used per display for MDP Comp.

Although 6 pipes are available in total but only up to 4 pipes are
available per display (single mixer) for MDP Composition.

CRs-Fixed: 450985

Change-Id: I99df91ab955a1a7110dfbea2aaf790ba26058a78
diff --git a/libhwcomposer/hwc_mdpcomp.h b/libhwcomposer/hwc_mdpcomp.h
index 9ae9b2e..45cd77b 100644
--- a/libhwcomposer/hwc_mdpcomp.h
+++ b/libhwcomposer/hwc_mdpcomp.h
@@ -25,6 +25,7 @@
 #include <overlay.h>
 
 #define DEFAULT_IDLE_TIME 2000
+#define MAX_PIPES_PER_MIXER 4
 
 namespace qhwc {
 namespace ovutils = overlay::utils;