drm/v3d: Don't set drm_device->dev_private

And switch the helper over to container_of, which is a bunch faster
than chasing a pointer. Plus allows gcc to see through this maze.

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/20200415074034.175360-8-daniel.vetter@ffwll.ch