drm/host1x: stop casting VMA offsets to 32bit

VMA offsets are 64bit so do not cast them to "unsigned int". Also remove
the (now useless) offset-retrieval helper. The VMA manager provides simple
enough helpers.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: "Terje Bergström" <tbergstrom@nvidia.com>
Cc: Arto Merilainen <amerilainen@nvidia.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>