nvc0: Fix xonotic bug

And remove 'bind parameters buffer setup' which generates the dmesg
error : nouveau E[  PGRAPH][0000:01:00.0] DATA_ERROR [INVALID_BITFIELD]
ch 3 [0x003fac5000 glxgears[6458]] subc 1 class 0x90c0 mthd 0x2380 data
0x02020000
nvc0: Factorize code between nve4_compute and nvc0_compute
nvc0: refactoring, factor out some code between nvc0 and e4