Linux 5.1.9 build failure with CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT=n

[Sven Joachim]

Commit 1e07d63749 ("drm/nouveau: add kconfig option to turn off nouveau
legacy contexts. (v3)") has caused a build failure for me when I
actually tried that option (CONFIG_NOUVEAU_LEGACY_CTX_SUPPORT=n):

,----
| Kernel: arch/x86/boot/bzImage is ready  (#1)
|   Building modules, stage 2.
|   MODPOST 290 modules
| ERROR: "drm_legacy_mmap" [drivers/gpu/drm/nouveau/nouveau.ko] undefined!
| scripts/Makefile.modpost:91: recipe for target '__modpost' failed
`----

Upstream does not have that problem, as commit bed2dd8421 ("drm/ttm:
Quick-test mmap offset in ttm_bo_mmap()") has removed the use of
drm_legacy_mmap from nouveau_ttm.c.  Unfortunately that commit does not
apply in 5.1.9.

Most likely 4.19.50 and 4.14.125 are also affected, I haven't tested
them yet.

Cheers,
       Sven