mesa: Build swrast gallium driver along with vc4

when running -ctestimage target, it fails because of errors it finds in
Xorg logs

| Log:
/mnt/a/yoe/build/tmp/work/raspberrypi3-yoe-linux-gnueabi/core-image-sato/1.0-r0/target_logs/Xorg.0.log
| -----------------------
| Central error: [    14.760] (EE) AIGLX error: dlopen of /usr/lib/dri/swrast_dri.so failed (/usr/lib/dri/swrast_dri.so: cannot open shared object file: No such file or directory)

Therefore we enable swrast gallium drivers to make xorg happy

Signed-off-by: Khem Raj <raj.khem@gmail.com>

recipes-graphics/mesa/mesa_%.bbappend

@@ -3,5 +3,5 @@
 # as default. To state out clearly that Raspi needs dri3 and to avoid surprises
 # in case oe-core changes this default, we set dri3 explicitly.
 PACKAGECONFIG_append_rpi = " gallium ${@bb.utils.contains('DISTRO_FEATURES', 'x11 opengl', 'x11 dri3', '', d)}"
-GALLIUMDRIVERS_rpi = "vc4"
+GALLIUMDRIVERS_append_rpi = ",vc4"
 DRIDRIVERS_rpi = ""