nvidia: prevent grub2-mkconfig removing nouveau blacklisting

Running 'grub2-mkconfig -o /boot/grub2/grub.cfg' causes the kernel boot
params nouveau.modeset=0 rd.driver.blacklist=nouveau to be removed.

This patch adds nouveau.modeset=0 rd.driver.blacklist=nouveau to the
default command line parameters so nouveau will be blacklisted by
default for all kernels.

See: http://elrepo.org/bugs/view.php?id=473

Signed-off-by: Philip J Perry <phil@elrepo.org>