drm/nouveau/fb: handle sysmem flush page from common code

- also executes pre-DEVINIT, so early boot is able to DMA sysmem

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>

drm/nouveau/fb: switch to instanced constructor

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>

drm/nouveau/mmu: remove old vmm frontend

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drm/nouveau/fb: finalise big page size selection in constructor

MMU will need to know this during its constructor, so we can't delay
deciding this until init-time.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drm/nouveau/fb: add gm20b device

gm20b's FB has the same capabilities as gm200, minus the ability to
allocate RAM. Create a device that reflects this instead of re-using the
gk20a device which may be incorrect.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-By: Karol Herbst <karolherbst@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>