Now that we have properly clipped coordinates in plane state structure,
use them.

This also fixes bug where source x and y were adjusted for negative
value, but width and height weren't. It wasn't discovered because
primary plane usually doesn't have negative coordinates.
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201060550.10392-13-jernej.skrabec@siol.net