drm/panel: lvds: Simplify mode parsing

The mode parsing is currently implemented in three steps:
of_get_display_timing() - DT panel-timing to struct display_timing
videomode_from_timing() - struct display_timing to struct videomode
drm_display_mode_from_videomode() - struct videomode to struct drm_display_mode

Replace all that with simple of_get_drm_panel_display_mode() call,
which already populates struct drm_display_mode and then duplicate
that mode in panel_lvds_get_modes() each time, since the mode does
not change.

Nice bonus is the bus_flags parsed by of_get_drm_panel_display_mode()
out of panel-timing DT node, which is used in subsequent patch to fix
handling of 'de-active' DT property.

Tested-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dmitry Osipenko <digetx@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
To: dri-devel@lists.freedesktop.org
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220401162154.295152-1-marex@denx.de