drm: Manage drm_gem_init with drmm_

We might want to look into pushing this down into drm_mm_init, but
that would mean rolling out return codes to a pile of functions
unfortunately. So let's leave that for now.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-24-daniel.vetter@ffwll.ch