drm: mxsfb: Fix NULL pointer dereference

drm: mxsfb: Fix NULL pointer dereference

[ Upstream commit 622c9a3a7868e1eeca39c55305ca3ebec4742b64 ]

mxsfb should not ever dereference the NULL pointer which
drm_atomic_get_new_bridge_state is allowed to return.
Assume a fixed format instead.

Fixes: b776b0f00f24 ("drm: mxsfb: Use bus_format from the nearest bridge if present")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220202081755.145716-3-alexander.stein@ew.tq-group.com
Signed-off-by: Sasha Levin <sashal@kernel.org>