sdm: Add support for hybrid composition

- Add interface for hybrid composition where some part
  of layer is composed by blit engine and rest by SDE.
- Use BlitEngine interface for the blit composition
- Allocate blit target buffers in Commit and use Reconfig to
  re-validate the Hybrid composition
- Add C2D as a BlitEngine to get layers composed on the blit
  target.

Change-Id: I849da73055920ab13c1a260c9511478457e6894b
diff --git a/sdm/include/utils/constants.h b/sdm/include/utils/constants.h
index bf5a916..9ea6e70 100644
--- a/sdm/include/utils/constants.h
+++ b/sdm/include/utils/constants.h
@@ -87,6 +87,7 @@
 
   const int kThreadPriorityUrgent = -9;
   const int kMaxRotatePerLayer = 2;
+  const uint32_t kMaxBlitTargetLayers = 2;
 
   typedef void * Handle;