Fix narrowing conversion between int <-> unscoped enum.

Bug: http://b/26747884
Change-Id: Ife86534c45642cdeb004e68b766c123a4131d6fe
diff --git a/gralloc.cpp b/gralloc.cpp
index 8162e91..421aeb5 100644
--- a/gralloc.cpp
+++ b/gralloc.cpp
@@ -62,7 +62,7 @@
 
 	va_start(args, op);
 	switch (op) {
-	case GRALLOC_MODULE_PERFORM_GET_DRM_FD:
+	case static_cast<int>(GRALLOC_MODULE_PERFORM_GET_DRM_FD):
 		{
 			int *fd = va_arg(args, int *);
 			*fd = gralloc_drm_get_fd(dmod->drm);