mesa-gl: Enable swrast dri backend for rpi/userland

After mesa switched build system to meson, the logic to generate dri
related artifacts changed too, which means when no dri backend is
enabled then dri drivers and corresponding headers dont get generated
and hence we end up with missing pkgconfig files e.g. dri.pc which
usually will come from full mesa3d package, but in rpi when userland is
used we only build GL pieces of mesa.

This patch therefore enables swrast dri backend when using userland,
which gives us the needed header and .pc files to build packages like
Xorg server, since EGL driver will come from userland, the dri backend
would be unused and hence should not be effective at runtime.

Addresses issue report with https://github.com/agherzan/meta-raspberrypi/pull/773

there is no need to check for graphics stack since mesa-gl is only used
with userland graphics

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Trevor Woerner <twoerner@gmail.com>
(cherry picked from commit 6c1c9ff4ee743cd90c4107e05be136dd4cb2dee6)
Signed-off-by: Ricardo Salveti <ricardo@foundries.io>

recipes-graphics/mesa/mesa-gl_%.bbappend

@@ -1,6 +1,8 @@
 PACKAGECONFIG_append_rpi = " gbm"
 PROVIDES_append_rpi = " virtual/libgbm"
 
+DRIDRIVERS_append_rpi = ",swrast"
+
 do_install_append_rpi() {
     rm -rf ${D}${includedir}/KHR/khrplatform.h
 }