drm/omap: fix possible NULL ref issue in tiler_reserve_2d

tiler_reserve_2d allocates memory but does not check if it got the
memory. Add the check and return ENOMEM on failure.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180329104038.29154-2-tomi.valkeinen@ti.com


Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>