drm/rockchip: add function to check if endpoint is a subdriver

To be able to have both internal subdrivers and external bridge
drivers as output endpoints of vops, add a function to be able
to distinguish these.

changes in v8:
- improved function documentation
- better error handling
- put calls for node and pdev references
changes in v6:
- added function to check subdriver vs. bridge

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Sandy Huang <hjc@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180830211207.10480-2-heiko@sntech.de