hwc: Query MDP capability for blending stages supported.

- Read MDP sys_fs node for blending stages supported per mixer
  instead of hard cording the value. This makes the implementation
  scalable across chipsets.
- Add binder support to change the max pipes per layer value
  dynamically for MDP comp.

Change-Id: I3b83f4bdc2e2e81ca55c54fc6bcfd61c55fef501
diff --git a/libhwcomposer/hwc_mdpcomp.h b/libhwcomposer/hwc_mdpcomp.h
index 4634fbc..7fa6674 100644
--- a/libhwcomposer/hwc_mdpcomp.h
+++ b/libhwcomposer/hwc_mdpcomp.h
@@ -25,8 +25,6 @@
 #include <cutils/properties.h>
 #include <overlay.h>
 
-#define MAX_PIPES_PER_MIXER 4
-
 namespace overlay {
 class Rotator;
 };
@@ -57,6 +55,7 @@
     static bool isIdleFallback() { return sIdleFallBack; }
     static void dynamicDebug(bool enable){ sDebugLogs = enable; }
     static void setIdleTimeout(const uint32_t& timeout);
+    static void setMaxPipesPerMixer(const uint32_t value);
 
 protected:
     enum { MAX_SEC_LAYERS = 1 }; //TODO add property support
@@ -104,7 +103,7 @@
 
         /* maps mdp list to layer list */
         int mdpCount;
-        struct PipeLayerPair mdpToLayer[MAX_PIPES_PER_MIXER];
+        struct PipeLayerPair mdpToLayer[MAX_NUM_BLEND_STAGES];
 
         /* layer composing on FB? */
         int fbCount;