drm/radeon: track global bo name and always return the same

To avoid kernel rejecting cs if we return different global name
for same bo keep track of global name and always return the same.
Seems to fix issue with suspend/resume failing and repeatly printing
following message :
[drm:radeon_cs_ioctl] *ERROR* Failed to parse relocation -35!

There might still be way for a rogue program to trigger this issue.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>