drm/i915: Refer to GGTT {,VM} consistently

Refer to the GGTT VM consistently as "ggtt->base" instead of just "ggtt",
"vm" or indirectly through other variables like "dev_priv->ggtt.base"
to avoid confusion with the i915_ggtt object itself and PPGTT VMs.

Refer to the GGTT as "ggtt" instead of indirectly through chaining.

As a bonus gets rid of the long-standing i915_obj_to_ggtt vs.
i915_gem_obj_to_ggtt conflict, due to removal of i915_obj_to_ggtt!

v2:
- Added some more after grepping sources with Chris

v3:
- Refer to GGTT VM through ggtt->base consistently instead of ggtt_vm
  (Chris)

v4:
- Convert all dev_priv->ggtt->foo accesses to ggtt->foo.

v5:
- Make patch checker happy

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/intel_overlay.c b/drivers/gpu/drm/i915/intel_overlay.c
index e1acb41..6694e92 100644
--- a/drivers/gpu/drm/i915/intel_overlay.c
+++ b/drivers/gpu/drm/i915/intel_overlay.c
@@ -190,13 +190,14 @@
 static struct overlay_registers __iomem *
 intel_overlay_map_regs(struct intel_overlay *overlay)
 {
-	struct drm_i915_private *dev_priv = overlay->dev->dev_private;
+	struct drm_i915_private *dev_priv = to_i915(overlay->dev);
+	struct i915_ggtt *ggtt = &dev_priv->ggtt;
 	struct overlay_registers __iomem *regs;
 
 	if (OVERLAY_NEEDS_PHYSICAL(overlay->dev))
 		regs = (struct overlay_registers __iomem *)overlay->reg_bo->phys_handle->vaddr;
 	else
-		regs = io_mapping_map_wc(dev_priv->ggtt.mappable,
+		regs = io_mapping_map_wc(ggtt->mappable,
 					 i915_gem_obj_ggtt_offset(overlay->reg_bo));
 
 	return regs;
@@ -1481,7 +1482,8 @@
 static struct overlay_registers __iomem *
 intel_overlay_map_regs_atomic(struct intel_overlay *overlay)
 {
-	struct drm_i915_private *dev_priv = overlay->dev->dev_private;
+	struct drm_i915_private *dev_priv = to_i915(overlay->dev);
+	struct i915_ggtt *ggtt = &dev_priv->ggtt;
 	struct overlay_registers __iomem *regs;
 
 	if (OVERLAY_NEEDS_PHYSICAL(overlay->dev))
@@ -1490,7 +1492,7 @@
 		regs = (struct overlay_registers __iomem *)
 			overlay->reg_bo->phys_handle->vaddr;
 	else
-		regs = io_mapping_map_atomic_wc(dev_priv->ggtt.mappable,
+		regs = io_mapping_map_atomic_wc(ggtt->mappable,
 						i915_gem_obj_ggtt_offset(overlay->reg_bo));
 
 	return regs;