eglimage: allow direct import of RGB dma-bufs

Add mapping between GStreamer and DRM format for direct import
of RGB pixel data.

Bug: 161191133
Change-Id: I6b2438f457bb38107b10e4bdfe175d83b4025800
diff --git a/gst-libs/gst/gl/egl/gsteglimage.c b/gst-libs/gst/gl/egl/gsteglimage.c
index 498c623..973f6af 100644
--- a/gst-libs/gst/gl/egl/gsteglimage.c
+++ b/gst-libs/gst/gl/egl/gsteglimage.c
@@ -512,6 +512,9 @@
     case GST_VIDEO_FORMAT_xBGR:
       return DRM_FORMAT_RGBX8888;
 
+    case GST_VIDEO_FORMAT_RGB:
+      return DRM_FORMAT_BGR888;
+
     default:
       GST_INFO ("Unsupported format for direct DMABuf.");
       return -1;