drm/selftests: Add drm_rect selftests

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>