drm: Add DRM_GEM_FOPS

The DEFINE_DRM_GEM_FOPS() helper is a bit limiting if a driver wants to
provide additional file ops, like show_fdinfo().

v2: Split out DRM_GEM_FOPS instead of making DEFINE_DRM_GEM_FOPS
    varardic
v3: nits

Signed-off-by: Rob Clark <robdclark@chromium.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Patchwork: https://patchwork.freedesktop.org/patch/488904/
Link: https://lore.kernel.org/r/20220609174213.2265938-1-robdclark@gmail.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>