drm/i915: stop assigning drm->dev_private pointer

We no longer need or use it as we subclass struct drm_device in our
struct drm_i915_private, and can always use to_i915() to get at
i915. Stop assigning the pointer to catch anyone trying to add new users
for ->dev_private.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Dale B Stimson <dale.b.stimson@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224113312.13674-1-jani.nikula@intel.com