nouveau: rework and simplify nv04/nv05 driver a bit

nouveau: rework and simplify nv04/nv05 driver a bit

TEXTURED_TRIANGLE and MULTITEX_TRIANGLE are both a bit special in that if
you use any other graph object in the meantime they'll forget their state
and spew a lovely METHOD_CNT error at you when you try to draw.

The pre-newlib driver has a flush_notify() hook which does this state
re-emit, and a number of random workarounds like extra flushes and state
dirtying after various operations to solve this issue.

I'm taking a slightly different approach to things instead, which has the
nice side-effect of removing the divergent code-paths for ttri/mtri, the
flush/dirty workarounds and the need for flush_notify.  Also gives a few
FPS boost in OA, yay.