drm: Try to document legacy DPMS uapi a bit better

drm: Try to document legacy DPMS uapi a bit better

Due to inconsistency of how various legacy drivers implemented DPMS
the DPMS uabi has a lot of quirks. Atomic standardizes this, but
drivers using the DPMS support can't rely on that since legacy drivers
still exist.

Laurent asked for this.

v2:
Improve commit message and explain that DPMS doesn't really exist for
the atomic ioctl (it's "ACTIVE" on the CRTC instead).

Text polish from Eric's review.

Cc: Laurent Pinchart <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Eric Anholt <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>
Reviewed-by: Sean Paul <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]