drm/i915: Use the correct size of the GTT for placing the per-process entries

The current layout is to place the per-process tables at the end of the
GTT. However, this is currently using a hardcoded maximum size for the GTT
and not taking in account limitations imposed by the BIOS. Use the value
for the total number of entries allocated in the table as provided by
the configuration registers.
Reported-by: NMatthew Garrett <mjg@redhat.com>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Matthew Garret <mjg@redhat.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -72,7 +72,7 @@ int i915_gem_init_aliasing_ppgtt(struct drm_device *dev)
 	/* ppgtt PDEs reside in the global gtt pagetable, which has 512*1024
 	 * entries. For aliasing ppgtt support we just steal them at the end for
 	 * now. */
-	first_pd_entry_in_global_pt = 512*1024 - I915_PPGTT_PD_ENTRIES;
+	first_pd_entry_in_global_pt = dev_priv->mm.gtt->gtt_total_entries - I915_PPGTT_PD_ENTRIES;
 
 	ppgtt = kzalloc(sizeof(*ppgtt), GFP_KERNEL);
 	if (!ppgtt)