drm/nv50: Remove redundant/incorrect ctxvals initialisation.

11c/004 offset corresponds to PGRAPH reg 0x400828, and is initialised
earlier anyway by both our ctxprog generator and blob ctxvals. It's
actually incorrect with the generator, since we use different layout
on pre-NVA0.

Signed-off-by: Marcin Kościelnicki <koriakin@0x04.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>