drm/i915/tasklet: separate local hacks around struct tasklet_struct

Add a dedicated file for the local functions around struct
tasklet_struct. Far from ideal, but better placed in a dedicated file
than i915_gem.h.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220607094207.536699-1-jani.nikula@intel.com

drm/i915: add gem/gt TODO

We've discussed a bit how to get the gem/gt team better integrated
and collaborate more with the wider community and agreed to the
following:

- all gem/gt patches are reviewed on dri-devel for now. That's
overkill, but in the past there was definitely too little of that.

- i915-gem folks are encouraged to cross review core patches from
other teams

- big features (especially uapi changes) need to be discussed in an
rfc patch that documents the interface and big picture design,
before we get lost in the details of the code

- Also a rough TODO (can be refined as we go ofc) to get gem/gt back
on track, like we've e.g. done with DAL/DC to get that in shape.

v2:
- add dma_fence annotations (Dave)
- tasklet helpers (Jani on irc)

There was also a discussion about moving these into gitlab issues, or
gitlab issues as additional discussion place at least. For now it's
just the TODO file

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Dave Airlie <airlied@redhat.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210324211041.1354941-1-daniel.vetter@ffwll.ch