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

commit cc5034a5 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: dd220a00

 ("drm/radeon/kms: add support for streamout v7")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>