drm/amdgpu: fix warning in ras_debugfs_create_all()

Fix the warning
"warn: variable dereferenced before check 'obj' (see line 1131)"
by removing unnecessary checks as amdgpu_ras_debugfs_create_all()
is only called from amdgpu_debugfs_init() where obj member in
con->head list is not NULL.
Use list_for_each_entry() instead list_for_each_entry_safe() as obj
do not to be freeing or removing from list during this process.

Signed-off-by: Stanley.Yang <Stanley.Yang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>