drm: fix typos in plane and CRTC overviews

Fix one missing letter, and one duplicate struct field reference.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/3gESG7rm6lCex6qBZnl3YXTfefKZODEvJ4DX4RoI1cY@cp4-web-040.plabs.ch