drm: Update documentation and TODO of gem_prime_mmap hook

The hook gem_prime_mmap in struct drm_driver is deprecated. Document
the new requirements.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211108102846.309-4-tzimmermann@suse.de