drm: Use the ERESTARTSYS error code

drm: Use the ERESTARTSYS error code

* Linux kernel functions can return -ERESTARTSYS in some cases, a
  negative value indicating an error (typically -512)

* Howewer, ERESTARTSYS was previously as ERESTART, itself defined
  as -1 in *BSD kernels

* The -ERESTARTSYS return value thus was positive on the DragonFly
  kernel, potentially breaking various drm error checks.

* Fix this issue by defining ERESTARTSYS separately

Thanks-to: Imre Vadasz for discovering the original problem