drm: warn if cursor plane is set with legacy funcs

A driver must not set drm_crtc.cursor and any of the legacy funcs at the
same time, otherwise it's not clear which one DRM core should use for
legacy cursor updates.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201222134002.161613-1-contact@emersion.fr