drm: etnaviv: Replace of_platform.h with explicit includes

Etnaviv doesn't use anything from of_platform.h, but depends on
of.h, of_device.h, and platform_device.h which are all implicitly
included, but that is going to be removed soon.

Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>