drm/bridge: analogix_dp: Fix incorrect operations with register ANALOGIX_DP_FUNC_EN_1

Register ANALOGIX_DP_FUNC_EN_1(offset 0x18), Rockchip is different to
Exynos:

on Exynos edp phy,
BIT 7		MASTER_VID_FUNC_EN_N
BIT 6		reserved
BIT 5		SLAVE_VID_FUNC_EN_N

on Rockchip edp phy,
BIT 7		reserved
BIT 6		RK_VID_CAP_FUNC_EN_N
BIT 5		RK_VID_FIFO_FUNC_EN_N

So, we should do some private operations to Rockchip.

Cc: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: zain wang <wzz@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423105003.9004-17-enric.balletbo@collabora.com