drm/i915: Rename GGTT init functions

Rename and document the GGTT init functions to give a better
idea of the context where they are called from.

i915_gem_gtt_init => i915_ggtt_init_hw
i915_gem_init_global_gtt => i915_gem_init_ggtt
i915_global_gtt_cleanup => i915_ggtt_cleanup_hw

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458830866-12578-1-git-send-email-joonas.lahtinen@linux.intel.com
diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.h b/drivers/gpu/drm/i915/i915_gem_gtt.h
index d804be0..242a13b 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.h
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.h
@@ -513,10 +513,9 @@
 		px_dma(ppgtt->base.scratch_pd);
 }
 
-int i915_gem_gtt_init(struct drm_device *dev);
-void i915_gem_init_global_gtt(struct drm_device *dev);
-void i915_global_gtt_cleanup(struct drm_device *dev);
-
+int i915_ggtt_init_hw(struct drm_device *dev);
+void i915_gem_init_ggtt(struct drm_device *dev);
+void i915_ggtt_cleanup_hw(struct drm_device *dev);
 
 int i915_ppgtt_init(struct drm_device *dev, struct i915_hw_ppgtt *ppgtt);
 int i915_ppgtt_init_hw(struct drm_device *dev);