drm/bridge: ti-sn65dsi86: Group DP link training bits in a function

We'll re-organize the ti_sn_bridge_enable() function a bit to group
together all the parts relating to link training and split them into a
sub-function.  This is not intended to have any functional change and
is in preparation for trying link training several times at different
rates.  One small side effect here is that if link training fails
we'll now leave the DP PLL disabled, but that seems like a sane thing
to do.

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.v3.7.I1fc75ad11db9048ef08cfe1ab7322753d9a219c7@changeid