bin/install_megadrivers: fix DESTDIR and -D*-path

This fixes -Ddri-drivers-path, -Dvdpau-libs-path, etc. with DESTDIR when
those paths are absolute. Currently due to the way python's os.path.join
handles absolute paths these will ignore DESTDIR, which is bad. This
fixes them to be relative to DESTDIR if that is set.

Fixes: 3218056e
       ("meson: Build i965 and dri stack")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
(cherry picked from commit ae3f45c1)