drm/rockchip: Implement mmap as GEM object function

Moving the driver-specific mmap code into a GEM object function allows
for using DRM helpers for various mmap callbacks.

The respective rockchip functions are being removed. The file_operations
structure fops is now being created by the helper macro
DEFINE_DRM_GEM_FOPS().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
[On rk3288 (pinky), rk3399 (gru-kevin, puma) and rk3328 (rock64)]
Tested-by: Heiko Stuebner <heiko@sntech.de>
[On RK3188/RK3066 (without iommu)]
Tested-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210624095502.8945-1-tzimmermann@suse.de