drm/radeon: cik_sdma_ctx_switch_enable() can be static

drivers/gpu/drm/radeon/cik_sdma.c:293:6: sparse: symbol 'cik_sdma_ctx_switch_enable' was not declared. Should it be static?

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>