We already have it in v3d_dev->drm.dev with zero additional pointer
chasing. Personally I don't like duplicated pointers like this
because:
- reviewers need to check whether the pointer is for the same or
different objects if there's multiple
- compilers have an easier time too

To avoid having to pull in some big headers I implemented the casting
function as a macro instead of a static inline. Typechecking thanks to
container_of still assured.

But also a bit a bikeshed, so feel free to ignore.

v2: More parens for v3d_to_pdev macro (checkpatch)
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-11-daniel.vetter@ffwll.ch