drm/nv50: fix a couple of vm init issues

Fixes overwriting the first page table entry when testing that the PRAMIN
BAR can be correctly read/written, and adds an additional bar flush after
poking the BAR3 control regs.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>