renderer/native: Also allow EGL_KHR_platform_gbm extension string

Proprietary drivers such as ARM Mali export EGL_KHR_platform_gbm instead
of EGL_MESA_platform_gbm. As such, GBM platform check should be done for
both MESA and non-MESA drivers.

https://bugzilla.gnome.org/show_bug.cgi?id=780668

src/backends/native/meta-renderer-native.c

@@ -1879,6 +1879,9 @@ init_gbm (MetaRendererNative *renderer_native,
 
   if (!meta_egl_has_extensions (egl, EGL_NO_DISPLAY, NULL,
                                 "EGL_MESA_platform_gbm",
+                                NULL) &&
+      !meta_egl_has_extensions (egl, EGL_NO_DISPLAY, NULL,
+                                "EGL_KHR_platform_gbm",
                                 NULL))
     {
       g_set_error (error, G_IO_ERROR,