nouveau: Fix build with CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT disabled

nouveau: Fix build with CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT disabled

Not-entirely-upstream-sha1-but-equivalent: bed2dd8421
("drm/ttm: Quick-test mmap offset in ttm_bo_mmap()")

Setting CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT=n (added by commit: b30a43ac7132)
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:
bed2dd8421 ("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 bed2dd8421.

NOTE. This is a minimal stable-only fix for trees where b30a43ac7132 is
backported as the build error affects nouveau only.

Fixes: b30a43ac7132 ("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>