drm/bridge: parade-ps8640: Use regmap APIs

Replace the direct i2c access (i2c_smbus_* functions) with regmap APIs,
which will simplify the future update on ps8640 driver.

Signed-off-by: Philip Chen <philipchen@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
[dianders: whitespace fixes reported by dim apply-branch]
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210921110556.v6.1.I2351df94f18d5d8debc22d4d100f36fac560409a@changeid