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

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>