configure.ac: default to disabled dri3 when --disable-dri is set

configure.ac: default to disabled dri3 when --disable-dri is set

Not too long ago, the dri3 code was living in src/glx, which in itself
was guarded by HAVE_DRI_GLX. As the name suggests we didn't dive into
the folder when dri was disabled, thus we missed that dri3 does not
consider/honour --enable-dri.

Cc: mesa-stable@lists.freedesktop.org
Fixes: 6bd9ba7d074 "loader: Add dri3 helper"
Cc: Pali Rohár <pali.rohar@gmail.com>
Reported-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>