drm/nouveau: nvkm: vmm: silence warning from cast

Cast the integer to a pointer-sized type first to keep the compiler
happy.

Fixes: 6b252cf4 ("drm/nouveau: nvkm/vmm: implement raw ops to manage uvmm")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230807163238.2091-3-dakr@redhat.com