drm/bridge: ti-sn65dsi86: Fix output polarity setting bug

According to the description in ti-sn65dsi86's datasheet:

CHA_HSYNC_POLARITY:
0 = Active High Pulse. Synchronization signal is high for the sync
pulse width. (default)
1 = Active Low Pulse. Synchronization signal is low for the sync
pulse width.

CHA_VSYNC_POLARITY:
0 = Active High Pulse. Synchronization signal is high for the sync
pulse width. (Default)
1 = Active Low Pulse. Synchronization signal is low for the sync
pulse width.

We should only set these bits when the polarity is negative.

Fixes: a095f15c ("drm/bridge: add support for sn65dsi86 bridge driver")
Signed-off-by: Qiqi Zhang <eddy.zhang@rock-chips.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221125104558.84616-1-eddy.zhang@rock-chips.com