libgralloc: Add support to custom buffer sizes.

Init allocSize hook function with alloc_size function

CRs-fixed: 379299
Change-Id: I514d47928f10b6c41404aa19d8b704f64327211b

Conflicts:
	libgralloc/gpu.cpp

libgralloc/gpu.cpp

@@ -43,6 +43,7 @@ gpu_context_t::gpu_context_t(const private_module_t* module,
     common.module  = const_cast<hw_module_t*>(&module->base.common);
     common.close   = gralloc_close;
     alloc          = gralloc_alloc;
+    allocSize      = gralloc_alloc_size;
     free           = gralloc_free;
 
 }