fbdev: fsl-diu-fb: Fix sparse warning due to virt_to_phys() prototype change

Explicitly cast __iomem pointer to const void* with __force to fix the
following warning:

   incorrect type in argument 1 (different address spaces)
      expected void const volatile *address
      got char [noderef] __iomem *screen_base

Signed-off-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311161120.BgyxTBMQ-lkp@intel.com/
Signed-off-by: Helge Deller <deller@gmx.de>