Fix bug caused by disabled gpu (#69)

Many AMD iGPUs are paired with a dGPU, and the BIOS has the option to
disable the iGPU. Doing so can fix a variety of software problems
related to multi-GPU-incompatible software. When this happens, rocm-smi
prints an error message to stderr since the amdgpu driver is not
initalized, but returns 0. Checking that there was actually data
returned fixes the crash (since amd-smi correctly returns 255) for those
with this hardware configuration.

Signed-off-by: Owen Hilyard <hilyard.owen@gmail.com>

Author: owenhilyard

mojo/mojo_host_platform.bzl

@@ -67,7 +67,7 @@ def _get_amd_constraints_with_rocm_smi(rctx, rocm_smi, gpu_mapping):
     _log_result(rctx, rocm_smi, result)
 
     constraints = []
-    if result.return_code == 0:
+    if result.return_code == 0 and len(result.stdout) > 0: #len(result.stdout) == 0 when the driver is not initialized
         blob = json.decode(result.stdout)
         if len(blob.keys()) == 0:
             fail("rocm-smi succeeded but didn't actually have any GPUs, please report this issue")