drm/nouveau: Implement mmap as GEM object function

Moving the driver-specific mmap code into a GEM object function allows
for using DRM helpers for various mmap callbacks.

The GEM object function is provided by GEM TTM helpers. Nouveau's
implementation of verify_access is unused and has been removed. Access
permissions are validated by the DRM helpers.

As a side effect, nouveau_ttm_vm_ops and nouveau_ttm_fault() are now
implemented in nouveau's GEM code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210525151055.8174-5-tzimmermann@suse.de