drm/arm: malidp: Use fourcc_mod_is_vendor() helper

Rather than open-coding the vendor extraction operation, use the newly
introduced helper macro.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210610111236.3814211-2-thierry.reding@gmail.com