drm_hwcomposer: add drm-mediatek to client-backend list

Since mediatek-drm has no support for premultiplied alpha buffers, we
can have issues with transparency.

Disable off-loading by adding "mediatek-drm" to the
client-backend list.

Change-Id: I12b034ecd5ae961264d01b6effaa4f4010036ac9
Suggested-by: Roman Stratiienko <r.stratiienko@gmail.com>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
diff --git a/backend/BackendManager.cpp b/backend/BackendManager.cpp
index b7601ee..0bacdcd 100644
--- a/backend/BackendManager.cpp
+++ b/backend/BackendManager.cpp
@@ -25,6 +25,7 @@
 
 const std::vector<std::string> BackendManager::client_devices_ = {
     "kirin",
+    "mediatek-drm",
 };
 
 BackendManager &BackendManager::GetInstance() {