drm: panel: simple: Warn in case of incorrect bus format for LVDS panels

Only the MEDIA_BUS_FMT_RGB666_1X7X3_SPWG,
MEDIA_BUS_FMT_RGB888_1X7X4_SPWG and MEDIA_BUS_FMT_RGB888_1X7X4_JEIDA bus
formats are valid for LVDS panels. Warn at probe time to catch the
common mistake of using an incorrect format, as well as discrepancies
between the bus format and the reported bpc.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200629233320.8774-5-laurent.pinchart+renesas@ideasonboard.com