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

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

drivers/gpu/drm/radeon/radeon_object.c: In function 'radeon_bo_unref':
drivers/gpu/drm/radeon/radeon_object.c:317:24: warning:
 variable 'rdev' set but not used [-Wunused-but-set-variable]

It not used  any more after commit
  e7e31600 ("drm/radeon: remove taking mclk_lock from radeon_bo_unref")

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