drm/ttm: Quick-test mmap offset in ttm_bo_mmap()

A BO's address has to be at least the minimum offset. Sharing this
test in ttm_bo_mmap() removes code from drivers. A full buffer-address
validation is still done within drm_vma_offset_lockup_locked().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/mgag200/mgag200_ttm.c

@@ -344,13 +344,8 @@ int mgag200_bo_push_sysram(struct mgag200_bo *bo)
 
 int mgag200_mmap(struct file *filp, struct vm_area_struct *vma)
 {
-	struct drm_file *file_priv;
-	struct mga_device *mdev;
+	struct drm_file *file_priv = filp->private_data;
+	struct mga_device *mdev = file_priv->minor->dev->dev_private;
 
-	if (unlikely(vma->vm_pgoff < DRM_FILE_PAGE_OFFSET))
-		return -EINVAL;
-
-	file_priv = filp->private_data;
-	mdev = file_priv->minor->dev->dev_private;
 	return ttm_bo_mmap(filp, vma, &mdev->ttm.bdev);
 }