drm/omap: fix possible NULL ref issue in tiler_reserve_2d

drm/omap: fix possible NULL ref issue in tiler_reserve_2d

[ Upstream commit 6a0f0c55619f0b82a677cab72e77c3444a5eee58 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>