drm/etnaviv: Fix off-by-one error in reloc checking

A relocation pointing to the last four bytes of a buffer can
legitimately happen in the case of small vertex buffers.

CC: stable@vger.kernel.org #4.9+
Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>