add support for debug.drm.mode

The mode string is "<xres>x<yres>[@<bpp>]".
diff --git a/gralloc_drm_intel.c b/gralloc_drm_intel.c
index 182b2c8..e21947a 100644
--- a/gralloc_drm_intel.c
+++ b/gralloc_drm_intel.c
@@ -496,7 +496,14 @@
 	struct drm_i915_getparam gp;
 	int pageflipping, id;
 
-	drm->fb_format = HAL_PIXEL_FORMAT_BGRA_8888;
+	switch (drm->fb_format) {
+	case HAL_PIXEL_FORMAT_BGRA_8888:
+	case HAL_PIXEL_FORMAT_RGB_565:
+		break;
+	default:
+		drm->fb_format = HAL_PIXEL_FORMAT_BGRA_8888;
+		break;
+	}
 
 	drm->mode_dirty_fb = 0;
 	/* why? */