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>