drm/gem: fix mmap vma size calculations

The VMA manager is page-size based so drm_vma_node_size() returns the size
in pages. However, drm_gem_mmap_obj() requires the size in bytes. Apply
PAGE_SHIFT so we no longer get EINVAL during mmaps due to too small
buffers.

This bug was introduced in commit:
  0de23977cfeb5b357ec884ba15417ae118ff9e9b
  "drm/gem: convert to new unified vma manager"

Fixes i915 gtt mmap failure reported by Sedat Dilek in:
  Re: linux-next: Tree for Jul 25 [ call-trace: drm | drm-intel related? ]

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>

drivers/gpu/drm/drm_gem.c

@@ -666,7 +666,7 @@ int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma)
 	}
 
 	obj = container_of(node, struct drm_gem_object, vma_node);
-	ret = drm_gem_mmap_obj(obj, drm_vma_node_size(node), vma);
+	ret = drm_gem_mmap_obj(obj, drm_vma_node_size(node) << PAGE_SHIFT, vma);
 
 	mutex_unlock(&dev->struct_mutex);