configure: bail out if building svga without libdrm

With recent commit we removed the NEED_NONNULL_WINSYS checks when
selecting the hardware (inc svga) winsys. svga has only one winsys
that explicitly requires libdrm (via it's bundled version of
vmwgfx_drm.h) but configure.ac never really checks for it.

Add the check early to prevent people from shooting themselves when
they select the driver but lack libdrm.

$ ./autogen.sh --disable-dri --disable-egl --disable-gallium-llvm
--with-dri-drivers=swrast --with-gallium-drivers=svga,swrast

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82539
Cc: "10.2 10.3" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
(cherry picked from commit 40bb6f9313)

configure.ac

@@ -1814,6 +1814,9 @@ if test -n "$with_gallium_drivers"; then
         case "x$driver" in
         xsvga)
             HAVE_GALLIUM_SVGA=yes
+            if test "x$have_libdrm" != xyes; then
+                AC_MSG_ERROR([Building svga requires libdrm >= $LIBDRM_REQUIRED])
+            fi
             GALLIUM_DRIVERS_DIRS="$GALLIUM_DRIVERS_DIRS svga softpipe"
             gallium_require_drm_loader
             gallium_check_st "svga/drm" "dri-vmwgfx" ""