nouveau: Fix build with CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT disabled

Not-entirely-upstream-sha1-but-equivalent: bed2dd84
("drm/ttm: Quick-test mmap offset in ttm_bo_mmap()")

Setting CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT=n (added by commit: b30a43ac)
causes the build to fail with:

ERROR: "drm_legacy_mmap" [drivers/gpu/drm/nouveau/nouveau.ko] undefined!

This does not happend upstream as the offending code got removed in:
bed2dd84 ("drm/ttm: Quick-test mmap offset in ttm_bo_mmap()")

Fix that by adding check for CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT around
the drm_legacy_mmap() call.

Also, as Sven Joachim pointed out, we need to make the check in
CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT=n case return -EINVAL as its done
for basically all other gpu drivers, especially in upstream kernels
drivers/gpu/drm/ttm/ttm_bo_vm.c as of the upstream commit bed2dd84.

NOTE. This is a minimal stable-only fix for trees where b30a43ac is
backported as the build error affects nouveau only.

Fixes: b30a43ac ("drm/nouveau: add kconfig option to turn off nouveau
       legacy contexts. (v3)")
Signed-off-by: Thomas Backlund <tmb@mageia.org>
Cc: stable@vger.kernel.org
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sven Joachim <svenjoac@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/nouveau/nouveau_ttm.c

@@ -169,7 +169,11 @@ nouveau_ttm_mmap(struct file *filp, struct vm_area_struct *vma)
 	struct nouveau_drm *drm = nouveau_drm(file_priv->minor->dev);
 
 	if (unlikely(vma->vm_pgoff < DRM_FILE_PAGE_OFFSET))
+#if defined(CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT)
 		return drm_legacy_mmap(filp, vma);
+#else
+		return -EINVAL;
+#endif
 
 	return ttm_bo_mmap(filp, vma, &drm->ttm.bdev);
 }