configure: enable dri3 only for linux

configure: enable dri3 only for linux

Currently only linux can make use of dri3, so it would make sense to
enable it explicitly for the platform.
Drop a duplicated libudev check while we're at it.

v3: Properly handle dri3 and reword commit message.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76377
Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>