drm/bridge: ti-sn65dsi83: halve horizontal syncs for dual LVDS output

drm/bridge: ti-sn65dsi83: halve horizontal syncs for dual LVDS output

Dual LVDS output (available on the SN65DSI84) requires HSYNC_PULSE_WIDTH
and HORIZONTAL_BACK_PORCH to be divided by two with respect to the values
used for single LVDS output.

While not clearly stated in the datasheet, this is needed according to the
DSI Tuner [0] output. It also makes sense intuitively because in dual LVDS
output two pixels at a time are output and so the output clock is half of
the pixel clock.

Some dual-LVDS panels refuse to show any picture without this fix.

Divide by two HORIZONTAL_FRONT_PORCH too, even though this register is used
only for test pattern generation which is not currently implemented by this
driver.

[0] https://www.ti.com/tool/DSI-TUNER

Fixes: ceb515ba29ba ("drm/bridge: ti-sn65dsi83: Add TI SN65DSI83 and SN65DSI84 driver")
Cc: stable@vger.kernel.org
Reviewed-by: Marek Vasut <marek.vasut@mailbox.org>
Link: https://patch.msgid.link/20260226-ti-sn65dsi83-dual-lvds-fixes-and-test-pattern-v1-2-2e15f5a9a6a0@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>