drm/nv50/gr: remove an unneeded write while initialising PGRAPH

The blob does not seem to write at that place for my NVAC, though it
does for my NV96, agreeing with what is done in the if/else structure
below. I guess someone forgot to remove the line when the if/else was
put in place.

Signed-off-by: Pierre Moreau <pierre.morrow@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>