drm/connector: Clarify the unit of TV margins

Commit 56406e15

 upstream.

All margins are expressed in pixels. Clarify that in the doc.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181206142439.10441-3-boris.brezillon@bootlin.com