drm/nouveau: Use drm_gem_ttm_dumb_map_offset()

Nouveau now uses drm_gem_ttm_dumb_map_offset() to implement
struct drm_driver.dumb_map_offset.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210408140139.27731-4-tzimmermann@suse.de