drm/i915: Move ringbuffer-related code to intel_ringbuffer.c.

This is preparation for supporting multiple ringbuffers on Ironlake.
The non-copy-and-paste changes are:
- de-staticing functions
- I915_GEM_GPU_DOMAINS moving to i915_drv.h to be used by both files.
- i915_gem_add_request had only half its implementation
  copy-and-pasted out of the middle of it.