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.
diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
index 9563901..da78f2c 100644
--- a/drivers/gpu/drm/i915/Makefile
+++ b/drivers/gpu/drm/i915/Makefile
@@ -22,6 +22,7 @@
 	  intel_fb.o \
 	  intel_tv.o \
 	  intel_dvo.o \
+	  intel_ringbuffer.o \
 	  intel_overlay.o \
 	  dvo_ch7xxx.o \
 	  dvo_ch7017.o \