nv50: keep track of PGRAPH state in nv50_screen

nv50: keep track of PGRAPH state in nv50_screen

Normally this is kept in nv50_context, and on switching the active
context, the state is copied from the previous context. However when the
last context is destroyed, this is lost, and a new context might later
be created. When the currently-active context is destroyed, save its
state in the screen, and restore it when setting the current context.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90363
Reported-by: Matteo Bruni <matteo.mystral@gmail.com>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Tested-by: Matteo Bruni <matteo.mystral@gmail.com>
Cc: mesa-stable@lists.freedesktop.org