drm/nv40: implement ctxprog/state generation

The context programs are *very* simple compared to the ones used by
the binary driver.  There's notes in nv40_grctx.c explaining most of
the things we don't implement.  If we discover if/why any of it is
required further down the track, we'll handle it then.

The PGRAPH state generated for each chipset should match what NVIDIA
do almost exactly (there's a couple of exceptions).  If someone has
a lot of time on their hands, they could figure out the mapping of
object/method to PGRAPH register and demagic the initial state a little,
it's not terribly important however.

At time of commit, confirmed to be working at least well enough for
accelerated X (and where tested, for 3D apps) on NV40, NV43, NV44, NV46,
NV49, NV4A, NV4B and NV4E.

A module option has been added to force the use of external firmware
blobs if it becomes required.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>