drm/imx: Make pipe number unsigned

drm/imx: Make pipe number unsigned

There's no reason whatsoever why this should ever be negative. The same
goes for the number of pipes added to the DRM device.

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450178476-26284-4-git-send-email-boris.brezillon@free-electrons.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>