amdgpu: Simplify error handling in parse_one_line

* Move empty/commented line check before the strdup and return -EAGAIN
  directly
* Initialize r = -EAGAIN and remove redundant assignments
* Set r = -ENOMEM if last strdup fails, and remove redundant goto

Acked-by: Slava Abramov <slava.abramov@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>