nvc0: keep track of PGRAPH state in nvc0_screen

nvc0: keep track of PGRAPH state in nvc0_screen

See identical commit for nv50. Destroying the current context and then
creating a new one or switching to another existing context would cause
the "current" state to not be properly initialized, so we save it off in
the screen.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org