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_utils.h b/libhwcomposer/hwc_utils.h
index 3f903b1..921c3e5 100644
--- a/libhwcomposer/hwc_utils.h
+++ b/libhwcomposer/hwc_utils.h
@@ -41,6 +41,7 @@
 #define LIKELY( exp )       (__builtin_expect( (exp) != 0, true  ))
 #define UNLIKELY( exp )     (__builtin_expect( (exp) != 0, false ))
 #define MAX_NUM_APP_LAYERS 32
+#define MAX_NUM_BLEND_STAGES 16
 #define MIN_DISPLAY_XRES 200
 #define MIN_DISPLAY_YRES 200
 #define HWC_WFDDISPSYNC_LOG 0