drm/radeon/radeon_ttm: Remove unused variable 'rbo' from radeon_bo_move()

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/radeon_ttm.c: In function ‘radeon_bo_move’:
 drivers/gpu/drm/radeon/radeon_ttm.c:201:27: warning: variable ‘rbo’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>