drm/gem-ttm-helper: Provide helper for struct drm_driver.dumb_map_offset

Provides an implementation of struct drm_driver.dumb_map_offset that
can be used by TTM-based GEM drivers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210408140139.27731-2-tzimmermann@suse.de