nv50/codegen: do not emitATOM() if the subOp is unknown

nv50/codegen: do not emitATOM() if the subOp is unknown

For debug build we'll hit the assert, for release we are going to emit random data
as subOp is used uninitilised. Spotted by gcc

codegen/nv50_ir_emit_nv50.cpp: In member function 'void nv50_ir::CodeEmitterNV50::emitATOM(const nv50_ir::Instruction*)':
codegen/nv50_ir_emit_nv50.cpp:1554:12: warning: 'subOp' may be used uninitialized in this function [-Wmaybe-uninitialized]
    uint8_t subOp;
            ^

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>