minigbm: Add minimal buffer allocation support for msm

Add minimal buffer allocation support using dumb_bo
for early enablement till the minigbm backend
implementation is avaialble for msm drv using
gem ioctls.

See CL:898422 for getting the DRV_MSM define in.

BUG=chromium:795946, b:72483556
TEST=Boot and see some graphics

Change-Id: Ieeb05923d756a3e4581a38cf6ce5cf406e2d197b
Reviewed-on: https://chromium-review.googlesource.com/897729
Commit-Ready: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
diff --git a/drv.c b/drv.c
index 7e12873..6d6e4fc 100644
--- a/drv.c
+++ b/drv.c
@@ -39,6 +39,9 @@
 #ifdef DRV_MEDIATEK
 extern const struct backend backend_mediatek;
 #endif
+#ifdef DRV_MSM
+extern const struct backend backend_msm;
+#endif
 extern const struct backend backend_nouveau;
 #ifdef DRV_RADEON
 extern const struct backend backend_radeon;
@@ -86,6 +89,9 @@
 #ifdef DRV_MEDIATEK
 		&backend_mediatek,
 #endif
+#ifdef DRV_MSM
+		&backend_msm,
+#endif
 		&backend_nouveau,
 #ifdef DRV_RADEON
 		&backend_radeon,