drm_gralloc: Enable allocation of flexible YUV420 buffers

This patch adds necessary switch cases to enable allocation of flexible
YUV420 buffers. They currently always map to NV12 buffers.

Change-Id: I57daba972698033240f9086b7badfad75335be2e
diff --git a/gralloc_drm.h b/gralloc_drm.h
index 3ad818f..f92ba5e 100644
--- a/gralloc_drm.h
+++ b/gralloc_drm.h
@@ -66,6 +66,7 @@
 	case HAL_PIXEL_FORMAT_YV12:
 	case HAL_PIXEL_FORMAT_YCbCr_422_SP:
 	case HAL_PIXEL_FORMAT_YCrCb_420_SP:
+	case HAL_PIXEL_FORMAT_YCbCr_420_888:
 		bpp = 1;
 		break;
 	default:
@@ -91,6 +92,7 @@
 		extra_height_div = 1;
 		break;
 	case HAL_PIXEL_FORMAT_YCrCb_420_SP:
+	case HAL_PIXEL_FORMAT_YCbCr_420_888:
 		align_w = 2;
 		align_h = 2;
 		extra_height_div = 2;