st/dri: define GALLIUM_SOFTPIPE when building kms_swrast

st/dri: define GALLIUM_SOFTPIPE when building kms_swrast

To avoid unresolved symbols in the DRI modules with earlier commit we
wrapped the innards of dri_kms_init_screen() in a
DRI_TARGET/GALLIUM_SOFTPIPE ifdef.

At the same time we forgot to adds the defines to the st/dri build
systems, breaking kms_swrast and gnome-continuous.

Drop the DRI_TARGET define, we're already in st/DRI.

Reported-by: Jasper St. Pierre <jstpierre@mecheye.net>
Reported-by: Vadim Rutkovsky <vrutkovs@redhat.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>