drm/bridge: ti-sn65dsi86: Simplify refclk handling

The clock framework makes it simple to deal with an optional clock.
You can call clk_get_optional() and if the clock isn't specified it'll
just return NULL without complaint. It's valid to pass NULL to
enable/disable/prepare/unprepare. Let's make use of this to simplify
things a tiny bit.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210416153909.v4.2.Ic9c04f960190faad5290738b2a35d73661862735@changeid