drm/fourcc: Fix fourcc_mod_code() definition

Avoid compiler warnings when the val parameter is an expression.

This is based on commit 5843f4e02fbe86a59981e35adc6cabebee46fdc0 from
Linux v4.16-rc1.

Acked-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>