nouveau: fix usage of PGRAPH_CTX_CONTROL on nv20+

http://gitweb.freedesktop.org/?p=mesa/drm.git;a=commitdiff;h=17985f07d68322519919a7f629a6d2d9bf3916ed could have broken some nvxx_graph code : it rename NV03_PGRAPH_CTX_CONTROL to NV10_PGRAPH_CTX_CONTROL, but forgot to update it in nvxx_graph file.

Also when migrating init stuff in http://gitweb.freedesktop.org/?p=mesa/drm.git;a=commitdiff;h=674cefd4fe4b537a20a10edcb4ec5df55facca8e, NV04_PGRAPH_CTX_CONTROL is used everywhere but the old ddx code use NV_PGRAPH_CTX_CONTROL_NV04 or NV_PGRAPH_CTX_CONTROL.