drm/tinydrm: Use struct drm_rect

This prepares for the switch to drm_atomic_helper_dirtyfb() in the next
patch. The damage helper returns a drm_rect so switch to that everywhere
including using a pointer in the dirty functions.

This is a non-functional change except for the debug print which looks a
bit different.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190115043643.2364-4-noralf@tronnes.org