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_fbc.c b/drivers/gpu/drm/i915/intel_fbc.c
index 2e571f5f..d5a7cfe 100644
--- a/drivers/gpu/drm/i915/intel_fbc.c
+++ b/drivers/gpu/drm/i915/intel_fbc.c
@@ -506,6 +506,7 @@
 				      int size,
 				      int fb_cpp)
 {
+	struct i915_ggtt *ggtt = &dev_priv->ggtt;
 	int compression_threshold = 1;
 	int ret;
 	u64 end;
@@ -516,9 +517,9 @@
 	 * underruns, even if that range is not reserved by the BIOS. */
 	if (IS_BROADWELL(dev_priv) ||
 	    IS_SKYLAKE(dev_priv) || IS_KABYLAKE(dev_priv))
-		end = dev_priv->ggtt.stolen_size - 8 * 1024 * 1024;
+		end = ggtt->stolen_size - 8 * 1024 * 1024;
 	else
-		end = dev_priv->ggtt.stolen_usable_size;
+		end = ggtt->stolen_usable_size;
 
 	/* HACK: This code depends on what we will do in *_enable_fbc. If that
 	 * code changes, this code needs to change as well.