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

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 <[email protected]>
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Signed-off-by: Helge Deller <[email protected]>