drm/vram-helper: cleanup drm_gem_vram_bo_driver_move_notify

Swapping bo->mem was completely unecessary. Cleanup the function which
is just a leftover from a TTM cleanup.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210211131659.276275-1-christian.koenig@amd.com