drm/gem-cma-helper: Remove drm_gem_cma_dumb_map_offset()

There are no more users of drm_gem_cma_dumb_map_offset(), so remove it.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1502034068-51384-20-git-send-email-noralf@tronnes.org