configure: use default dri drivers whenever opengl and dri are enabled

Commit ee55500c22a(configure: cleanup classic dri drivers handling)
cleaned up the logic handling autodetection of dri drivers, but missed
the case when one can explicitly disable dri, and still request opengl.

Fixes build issues for the following
./autogen.sh --disable-dri --with-gallium-drivers=swrast

While we're here, explicitly clear with_dri_drivers whenever building
without such drivers to prevent choking later on.

v2: Simplify with_dri_drivers handling.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75126
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

configure.ac

@@ -950,15 +950,13 @@ AC_ARG_WITH([dri-drivers],
     [with_dri_drivers="$withval"],
     [with_dri_drivers=auto])
 
-case "$with_dri_drivers" in
-no) ;;
-auto)
-    # classic DRI drivers
-    if test "x$enable_opengl" = xyes; then
+if test "x$with_dri_drivers" = xauto; then
+    if test "x$enable_opengl" = xyes -a "x$enable_dri" = xyes; then
         with_dri_drivers="yes"
+    else
+        with_dri_drivers="no"
     fi
-    ;;
-esac
+fi
 
 dnl If $with_dri_drivers is yes, drivers will be added through
 dnl platform checks. Set DEFINES and LIB_DEPS
@@ -1039,7 +1037,7 @@ AC_SUBST([GALLIUM_DRI_LIB_DEPS])
 
 DRI_DIRS=""
 dnl Duplicates in DRI_DIRS are removed by sorting it after this block
-if test -n "$with_dri_drivers"; then
+if test "x$with_dri_drivers" != xno; then
     if test "x$enable_opengl" != xyes; then
         AC_MSG_ERROR([--with-dri-drivers requires OpenGL])
     fi