media: img: vxd: Drop the unneeded umin() function

The stable commit 8d8be62a ("minmax: add umin(a, b) and umax(a, b)")
introduced an umin() macro. This conflicts with the umin() inline
function in vxe-vxd driver. So drop this inline function to fix the
build error.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>