drm/bridge: lt9611: fix clock calculation

Instead of having several fixed values for the pcr register, calculate
it before programming. This allows the bridge to support most of the
display modes.

Fixes: 23278bf5 ("drm/bridge: Introduce LT9611 DSI to HDMI bridge")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>