drm/hisilicon: Don't set drm_device->platformdev

It's deprecated and only should be used by drivers which still use
drm_platform_init, not by anyone else.

And indeed it's entirely unused and can be nuked.

This required a bit more fudging, but I guess kirin_dc_ops really
wants to operate on the platform_device, not something else. Also
bonus points for implementing abstraction, and then storing the vfunc
in a global variable.

Cc: Xinliang Liu <xinliang.liu@linaro.org>
Cc: Xinwei Kong <kong.kongxinwei@hisilicon.com>
Cc: Archit Taneja <architt@codeaurora.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-7-git-send-email-daniel.vetter@ffwll.ch