drm/selftests: Add drm_rect selftests

Add selftests for drm_rect. A few basic ones for clipped and unclipped
cases, and a few special ones for specific bugs we had in the code.

I'm too lazy to think of more corner cases to check at this time.
Maybe later.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191122175623.13565-5-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@st.com>
diff --git a/include/drm/drm_rect.h b/include/drm/drm_rect.h
index cd01061..57a3be9 100644
--- a/include/drm/drm_rect.h
+++ b/include/drm/drm_rect.h
@@ -24,6 +24,8 @@
 #ifndef DRM_RECT_H
 #define DRM_RECT_H
 
+#include <linux/types.h>
+
 /**
  * DOC: rect utils
  *