drm/omap: dsi: drop custom panel capability support

Due to previous changes the DSI encoder gets the capabilities
via DSI client's mode_flags and no longer needs the omapdss
specific caps. The core code now checks if the DSI encoder
is actually configured into command mode instead of just checking
the panel capabilities.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-32-tomi.valkeinen@ti.com