drm/managed: Cleanup of unused functions and polishing docs

Following functions are only used internally, not by drivers:
- devm_drm_dev_init

Also, now that we have a very slick and polished way to allocate a
drm_device with devm_drm_dev_alloc, update all the docs to reflect the
new reality. Mostly this consists of deleting old and misleading
hints. Two main ones:

- it is no longer required that the drm_device base class is first in
  the structure. devm_drm_dev_alloc can cope with it being anywhere

- obviously embedded now strongly recommends using devm_drm_dev_alloc

v2: Fix typos (Noralf)

v3: Split out the removal of drm_dev_init, that's blocked on some
discussions on how to convert vgem/vkms/i915-selftests. Adjust commit
message to reflect that.

Cc: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Noralf Trønnes <noralf@tronnes.org> (v2)
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Luben Tuikov <luben.tuikov@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200902072627.3617301-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_managed.c

@@ -27,7 +27,7 @@
  * be done directly with drmm_kmalloc() and the related functions. Everything
  * will be released on the final drm_dev_put() in reverse order of how the
  * release actions have been added and memory has been allocated since driver
- * loading started with drm_dev_init().
+ * loading started with devm_drm_dev_alloc().
  *
  * Note that release actions and managed memory can also be added and removed
  * during the lifetime of the driver, all the functions are fully concurrent