rockchip: video: vop: Use endpoint compatible string to find VOP mode

rockchip: video: vop: Use endpoint compatible string to find VOP mode

The current code is using an hard coded enum and the of node reg value of
endpoint to find out if the endpoint is mipi/hdmi/lvds/edp/dp. The order
is different between rk3288, rk3399 vop little, rk3399 vop big.

A possible solution would be to make sure that the rk3288.dtsi and
rk3399.dtsi files have "expected" reg value or an other solution is
to find the kind of endpoint by comparing the endpoint compatible value.

This patch is implementing the more flexible second solution.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Tested-by: Peter Robinson <pbrobinson@gmail.com>