drm/bridge: tc358775: fix support for jeida-18 and jeida-24

The bridge always uses 24bpp internally. Therefore, for jeida-18
mapping we need to discard the lowest two bits for each channel and thus
starting with LV_[RGB]2. jeida-24 has the same mapping but uses four
lanes instead of three, with the forth pair transmitting the lowest two
bits of each channel. Thus, the mapping between jeida-18 and jeida-24
is actually the same, except that one channel is turned off (by
selecting the RGB666 format in VPCTRL).

While at it, remove the bogus comment about the hardware default because
the default is overwritten in any case.

Tested with a jeida-18 display (Evervision VGG644804).

Fixes: b2697559 ("display/drm/bridge: TC358775 DSI/LVDS driver")
Signed-off-by: Michael Walle <mwalle@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240225062008.33191-5-tony@atomide.com