drm/vc4: Take margin setup into account when updating planes

Applyin margins is just a matter of scaling all planes appropriately
and adjusting the CRTC X/Y offset to account for the
left/right/top/bottom borders.

Create a vc4_plane_margins_adj() function doing that and call it from
vc4_plane_setup_clipping_and_scaling() so that we are ready to attach
margins properties to the HDMI connector.

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/20181206142439.10441-5-boris.brezillon@bootlin.com