drm: re-add deleted doc for drm_gem_open_ioctl

Add back the removed documentation for drm_gem_open_ioctl.
This patch is submitted in response to [1].

[1] https://lore.kernel.org/linux-arm-msm/20200728085244.GY6419@phenom.ffwll.local/

Signed-off-by: Steve Cohen <cohens@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1596000952-27621-1-git-send-email-cohens@codeaurora.org