drm/rockchip: Do not use memcpy for MMIO addresses

memcpy is only meant to be used for memory, and only that.
MMIO accessors should be used to access MMIO regions, preferably
the ones that correspond to the size of the register accessed.

Let's convert the bulk register copy to writel/readl_relaxed,
which is the correct API.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180220130120.5254-3-marc.zyngier@arm.com