fix building issues of 64-bit targets

Just change the 'data' field of gralloc_drm_handle_t
to be a pointer to struct gralloc_drm_bo_t.

Fix some warnings as well.
diff --git a/gralloc_drm.c b/gralloc_drm.c
index bd38d57..31ac988 100644
--- a/gralloc_drm.c
+++ b/gralloc_drm.c
@@ -211,10 +211,10 @@
 		}
 
 		handle->data_owner = gralloc_drm_get_pid();
-		handle->data = (int) bo;
+		handle->data = bo;
 	}
 
-	return (struct gralloc_drm_bo_t *) handle->data;
+	return handle->data;
 }
 
 /*
@@ -296,7 +296,7 @@
 	bo->refcount = 1;
 
 	handle->data_owner = gralloc_drm_get_pid();
-	handle->data = (int) bo;
+	handle->data = bo;
 
 	return bo;
 }
@@ -365,7 +365,7 @@
 	uint32_t *pitches, uint32_t *offsets, uint32_t *handles)
 {
 	struct gralloc_drm_handle_t *handle = gralloc_drm_handle(_handle);
-	struct gralloc_drm_bo_t *bo = (struct gralloc_drm_bo_t *) handle->data;
+	struct gralloc_drm_bo_t *bo = handle->data;
 	struct gralloc_drm_t *drm = bo->drm;
 
 	/* if handle exists and driver implements resolve_format */