drm/nouveau/tmr: fix nvkm_usec/nvkm_msec definitions

nvkm_timer_wait_init() takes a u64 as a duration parameter, but the
expression "(m) * 1000" will be promoted only to a 32-bit integer,
if 'm' is also an integer.  Changing the 1000 to 1000ULL ensures that
the expression will be 64 bits.

This change currently has no effect as there are no callers of
nvkm_msec() that exceed 2000ms.

Signed-off-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>