drm/etnaviv: fix dumping of iommuv2

etnaviv_iommuv2_dump_size(..) returns the number of PTE * SZ_4K but
etnaviv_iommuv2_dump(..) increments buf pointer even if there is no PTE.
This results in a bad buf pointer which gets used for memcpy(..), when
copying the MMU state in the coredump buffer.

Fixes: afb7b3b1 ("drm/etnaviv: implement IOMMUv2 translation")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>