drm/nouveau/instmem/gk20a: set DMA mask early

DMA mask is typically set in nouveau_ttm_init(), but this function is
called late during initialization and GK20A's instmem will have called
DMA functions before this happens.

Having a wrongly set DMA mask can result in the use of unneeded bounce
buffers. Set it early to avoid this.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>