drm/nouveau: Don't clear AGPCMD completely on INIT_RESET.

We just need to clear the SBA and ENABLE bits to reset the AGP
controller: If the AGP bridge was configured to use "fast writes",
clearing the FW bit would break the subsequent MMIO writes and
eventually end with a lockup.

Note that all the BIOSes I've seen do the same as we did (it works for
them because they don't use MMIO), OTOH the blob leaves FW untouched.
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_bios.c

@@ -2146,7 +2146,8 @@ init_reset(struct nvbios *bios, uint16_t offset, struct init_exec *iexec)
 	/* no iexec->execute check by design */
 
 	pci_nv_19 = bios_rd32(bios, NV_PBUS_PCI_NV_19);
-	bios_wr32(bios, NV_PBUS_PCI_NV_19, 0);
+	bios_wr32(bios, NV_PBUS_PCI_NV_19, pci_nv_19 & ~0xf00);
+
 	bios_wr32(bios, reg, value1);
 
 	udelay(10);