drm/bridge: ps8640: Print an error if VDO control fails

Print an error message inside ps8640_bridge_vdo_control() function when
it fails so we can simplify a bit the callers, they will only need to
check the error code.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Bilal Wasim <bwasim.lkml@gmail.com>
Tested-by: Bilal Wasim <bwasim.lkml@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826081526.674866-5-enric.balletbo@collabora.com