[PATCH] nouveau: reset AGP on init for < nv40

This is necessary for AGP to work after running bios init scripts on nv3x, and
is seen in mmio traces of all cards (nv04-nv4x)

I'm not making the equivalent change to nv40_mc.c, as early cards (6200, 6800gt)
use the 0x000018XX PBUS and later cards use the 0x000880XX PBUS and I don't know
the effects of using the wrong one