drm/ttm: fix mmap refcounting

When mapping ttm objects via drm_gem_ttm_mmap() helper
drm_gem_mmap_obj() will take an object reference.  That gets
never released due to ttm having its own reference counting.

Fix that by dropping the gem object reference once the ttm mmap
completed (and ttm refcount got bumped).

For that to work properly the drm_gem_object_get() call in
drm_gem_ttm_mmap() must be moved so it happens before calling
obj->funcs->mmap(), otherwise the gem refcount would go down
to zero.

Fixes: 231927d9 ("drm/ttm: add drm_gem_ttm_mmap()")
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20191113135612.19679-1-kraxel@redhat.com