drm/bridge: ti-sn65dsi86: Move panel_prepare() to pre_enable()

prepare() is the old-timey way to say pre_enable(). It should be called
before modeset. This fixes an issue where the panel on cheza must have
the regulator always-on/boot-on for it to work.

Changes in v3:
- Added to the set

Cc: Sandeep Panda <spanda@codeaurora.org>
Reviewed-by: Sandeep Panda <spanda@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180813213058.184821-5-sean@poorly.run