drm/encoder: Fix possible_crtcs documentation

Similar to possible_clones, we don't actually use possible_crtcs until
the driver is registered with userspace. So, fix the documentation to
indicate this.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190913222704.8241-3-lyude@redhat.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>