drm/radeon/evergreen_cs: fix missing break in switch statement

drm/radeon/evergreen_cs: fix missing break in switch statement

commit cc5034a5d293dd620484d1d836aa16c6764a1c8c upstream.

Add missing break statement in order to prevent the code from falling
through to case CB_TARGET_MASK.

This bug was found thanks to the ongoing efforts to enable
-Wimplicit-fallthrough.

Fixes: dd220a00e8bd ("drm/radeon/kms: add support for streamout v7")
Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>