drm/vram: switch vram helper to &drm_gem_object_funcs.mmap()

Wire up the new drm_gem_ttm_mmap() helper function,
use generic drm_gem_mmap for &fops.mmap and
delete dead drm_vram_mm_file_operations_mmap().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20191016115203.20095-10-kraxel@redhat.com