nv50/nvc0: make transfers aware of PIPE_TRANSFER_MAP_DIRECTLY

nv50/nvc0: make transfers aware of PIPE_TRANSFER_MAP_DIRECTLY

If state tracker asked us to map resource directly and we can't
do it (because of tiling), return NULL instead of doing full transfer
- state tracker should handle it and fallback to some other method
or repeat transfer without PIPE_TRANSFER_MAP_DIRECTLY.

It greatly improves performance of xorg state tracker on nv50+,
because its fallback (DFS/UTS) is much faster than full transfer.