revive vga@isa and pcdisplay for i386 INSTALL kernel

[Rin Okuyama <rokuyama%rk.phys.keio.ac.jp@localhost>]

Hi,

install/52845 revealed that we do not distribute floppy images of
kernel with vga@isa and pcdisplay for i386:

http://gnats.netbsd.org/52845

Why don't we add these devices into INSTALL kernel for i386?

They have been removed from GENERIC because they spoil DRMKMS drivers:

http://gnats.netbsd.org/49290

If I understand the PR correctly, there should be no problem to add
them into INSTALL, for which DRMKMS drivers are disabled.

Also, for -current and netbsd-8, nouveau is accidentally enabled for
INSTALL. It seems to be left forgotten when it got enabled for GENERIC.
I will disable it if there is no objection (also for amd64).

Thanks,
rin