drm/rockchip: Implement CRC debugfs API

Implement the .set_crc_source() callback and call the DP helpers
accordingly to start and stop CRC capture.

This is only done if this CRTC is currently using the eDP connector.

v3: Remove superfluous check on rockchip_crtc_state->output_type

v6: Remove superfluous variable

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303133936.14964-5-tomeu.vizoso@collabora.com