drm/amdgpu: fix a missing break in gfx_v11_0_handle_priv_fault

Fixes the following w1 warning:

drivers/gpu/drm/amd/amdgpu/gfx_v11_0.c:5873:2: warning: unannotated
fall-through between switch labels [-Wimplicit-fallthrough].

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