drm/bridge: ti-sn65dsi86: Split the setting of the dp and dsi rates

These two things were in one function.  Split into two.  This looks
like it's duplicating some code, but don't worry.  This is is just in
preparation for future changes.

This is intended to have zero functional change and will just make
future patches easier to understand.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191218143416.v3.1.Icb765d5799e9651e5249c0c27627ba33a9e411cf@changeid