r600g/cayman: handle empty vertex shaders

r600g/cayman: handle empty vertex shaders

Some of the geom shader tests produce an empty vertex shader,
on cayman we'd crash in the finaliser because last_cf was NULL.

cayman doesn't need the NOP workaround, so if the code arrives
here with no last_cf, just emit an END.

fixes crashes in a bunch of piglit geom shader tests.

Cc: "10.3 10.4" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Glenn Kennard <glenn.kennard@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>