drm/nouveau: Fix crtc->primary->fb vs. drm_fb fail

drm/nouveau: Fix crtc->primary->fb vs. drm_fb fail

So it looks like the code is trying to pick between the passed in fb and
crtc->primary->fb based on that funky 'bool atomic'. But later it will
mix uses of both drm_fb (which was picked by the aforementioned logic)
and crtc->primary->fb. So looks like a bug to me. Let's make it use
drm_fb only.

Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479498793-31021-11-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>