drm/radeon: Add break to switch statement in radeonfb_create_pinned_object()

Clang + -Wimplicit-fallthrough warns:

drivers/gpu/drm/radeon/radeon_fb.c:170:2: warning: unannotated
fall-through between switch labels [-Wimplicit-fallthrough]
        default:
        ^
drivers/gpu/drm/radeon/radeon_fb.c:170:2: note: insert 'break;' to avoid
fall-through
        default:
        ^
        break;
1 warning generated.

Clang's version of this warning is a little bit more pedantic than
GCC's. Add the missing break to satisfy it to match what has been done
all over the kernel tree.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>