drm/amdgpu: make gfx_v11_0_rlc_stop static

This symbol is not used outside of gfx_v11_0.c, so marks it static.

Fixes the following w1 warning:

drivers/gpu/drm/amd/amdgpu/gfx_v11_0.c:1945:6: warning: no previous
prototype for function 'gfx_v11_0_rlc_stop' [-Wmissing-prototypes].

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: sunliming <sunliming@kylinos.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>