r600g: use a single env var R600_DEBUG, disable bytecode dumping

r600g: use a single env var R600_DEBUG, disable bytecode dumping

Only the disassembler is used to dump shaders. Here's a few examples
how to use R600_DEBUG.

Log compute info:
  R600_DEBUG=compute

Dump all shaders:
  R600_DEBUG=fs,vs,gs,ps,cs

Dump pixel shaders only:
  R600_DEBUG=ps

Disable Hyper-Z:
  R600_DEBUG=nohyperz

Disable the LLVM backend:
  R600_DEBUG=nollvm

Or use any combination of the above, or print all options:
  R600_DEBUG=help

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>