drm: Mention the power state requirement on side-channel operations

The drm_connector detect, drm_dp_aux transfer and mipi_dsi_host
operations typically require to access their underlying device to
perform what is expected of them.

However, there's no guarantee on the fact that the device has been
enabled through atomic_enable or similar that will usually power the
device. The access to an unpowered device is then an undefined behaviour
ranging from the access being ignored to a hard CPU hang.

Let's document that expectation to avoid as much as possible those
consequences.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210616141529.630719-3-maxime@cerno.tech