nv30, nv40: non-trivially unify nv[34]0_state_fb.c

nv30, nv40: non-trivially unify nv[34]0_state_fb.c

The files are significantly different due to:
1. nv30 support 2 render targets, nv40 4
2. z-buffer pitch is set differently
3. nv30 has a limitation of colour_bits >= zeta_bits. This may not
   actually exist in the driver though
4. nv30 points color0 at depth in the depth-only case
5. nv30 sets NV34TCL_VIEWPORT_TX_ORIGIN to 0. This is probably
   unnecessary

This patch attempts to unify the two files and preserve the existing
behavior.