drm/etnaviv: Add helper functions to create and destroy platform device

drm/etnaviv: Add helper functions to create and destroy platform device

The newly introduced functions are etnaviv_create_platform_device() and
etnaviv_destroy_platform_device(). Those two function are pure function
and can be shared for other use case. Currently, the benefit is that we
no longer need to call of_node_put() for three different cases, we only
need to call it once in the etnaviv_init() function.

Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>