drm/ttm: Remove ttm_bo_mmap() and friends

The function ttm_bo_mmap is unused. Remove it and it's helpers; including
the verify_access callback in struct ttm_device_funcs.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210525151055.8174-8-tzimmermann@suse.de