gl-renderer: set num_images after import_simple_dmabuf

we have to set num_images after import_simple_dmabuf
call. Otherwise, egl_images will not be correctly
referenced in gl_renderer_attach_dmabuf.

(Found by clang source code analyzer)

Signed-off-by: Emre Ucan <eucan@de.adit-jv.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

libweston/gl-renderer.c

@@ -2216,6 +2216,7 @@ import_known_dmabuf(struct gl_renderer *gr,
 		image->images[0] = import_simple_dmabuf(gr, &image->dmabuf->attributes);
 		if (!image->images[0])
 			return false;
+		image->num_images = 1;
 		break;
 
 	case IMPORT_TYPE_GL_CONVERSION: