drm/ttm: remove set but not used variable 'rdev'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/radeon/radeon_ttm.c: In function 'radeon_move_vram_ram':
drivers/gpu/drm/radeon/radeon_ttm.c:254:24: warning:
 variable 'rdev' set but not used [-Wunused-but-set-variable]

drivers/gpu/drm/radeon/radeon_ttm.c: In function 'radeon_move_ram_vram':
drivers/gpu/drm/radeon/radeon_ttm.c:301:24: warning:
 variable 'rdev' set but not used [-Wunused-but-set-variable]

It's not used since a987fcaa ("ttm: Make parts of a struct ttm_bo_device
global.")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>