drm/bridge: tc358768: Add atomic_get_input_bus_fmts() implementation

Add atomic_get_input_bus_fmts() implementation, tc358768 has a parallel
RGB input interface with the actual bus format depending on the amount
of parallel input data lines.

Without this change when the tc358768 is used with less than 24bit the
color mapping is completely wrong.

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230330095941.428122-7-francesco@dolcini.it