Revert "drm/nouveau: add kconfig option to turn off nouveau legacy contexts. (v3)"

This reverts commit 61038233 which is
commit b30a43ac7132cdda833ac4b13dd1ebd35ace14b7 upstream.

Sven reports:
	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.

The ensuing discussion proposed a number of one-off patches, but no
solid agreement was made, so just revert the commit for now to get
people's systems building again.

Reported-by: Sven Joachim <svenjoac@gmx.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Thomas Backlund <tmb@mageia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>