drm/rect: Keep the clipped dst rectangle in place

drm/rect: Keep the clipped dst rectangle in place

Now that we've constrained the clipped source rectangle such
that it can't have negative dimensions doing the same for the
dst rectangle seems appropriate. Should at least result in
the clipped src and dst rectangles being a bit more consistent
with each other.

Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191122175623.13565-4-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@st.com>