drm/bridge: ti-sn65dsi86: Set max register for regmap

Set the maximum register to 0xff so we can dump the registers for this
device in debugfs.

Fixes: a095f15c

 ("drm/bridge: add support for sn65dsi86 bridge driver")
Cc: Rob Clark <robdclark@chromium.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211215002529.382383-1-swboyd@chromium.org