nvc0: reintroduce BGRA4 format support

nvc0: reintroduce BGRA4 format support

Commit 342e68dc60 (nvc0: remove BGRA4 format support) removed the
support to fix a WoW trace. However after further experimentation, I was
able to get the blit to work by using a different "fake" format in the
2d engine.

The reason why this worked on nv50 is that nv50 falls back to the 3d
blit path in case either the src or the dst aren't "faithfully"
supported, while nvc0 only does it for the dst format. RG8 is better
supported by the nvc0 2d engine than R16.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>