util/grub-install: Move platdir path canonicalization after files were copied to...

util/grub-install: Move platdir path canonicalization after files were copied to grubdir

The commit 3f9eace2d (util/grub-install: Delay copying files to
{grubdir,platdir} after install_device was validated) delaying
copying of files caused a regression when installing without an
existing directory structure.

This patch ensures that the platform directory actually exists by the
time the code tries to canonicalize its filename.

Fixes: 3f9eace2d (util/grub-install: Delay copying files to {grubdir,platdir} after install_device was validated)
Signed-off-by: Mate Kukri <mate.kukri@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>