drm/shmem: switch shmem helper to &drm_gem_object_funcs.mmap

Switch gem shmem helper to the new mmap() workflow,
from &gem_driver.fops.mmap to &drm_gem_object_funcs.mmap.

v2: Fix vm_flags and vm_page_prot handling.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20191016115203.20095-3-kraxel@redhat.com