drm2: remove DRM_SPINXXX macros

drm2: remove DRM_SPINXXX macros

* They did not apply 1:1 to DragonFly locking directives; keeping them
  in the drivers would have meant lockmgr locks would have had to be be
  used in situations where spinlocks could have done the job.

* Besides the obvious performance pessimization issues, they had the
  unfortunate effect of obscuring the source code

* The original DRM code in the Linux kernel doesn't even use them, they
  were purely a FreeBSD addition