drm/amdgpu/gfx11: Add missing break

This stops clang complaining:

drivers/gpu/drm/amd/amdgpu/gfx_v11_0.c:5895:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
        default:
        ^
drivers/gpu/drm/amd/amdgpu/gfx_v11_0.c:5895:2: note: insert 'break;' to avoid fall-through
        default:
        ^
        break;

Signed-off-by: Mike Lothian <mike@fireburn.co.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>