drm/sti: Add __iomem for mixer_dbg_mxn's parameter

Sparse complains about incorrect type in argument 1.
expected void const volatile  __iomem *ptr but got void *.
so modify mixer_dbg_mxn's addr parameter.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202411191809.6V3c826r-lkp@intel.com/


Fixes: a5f81078 ("drm/sti: add debugfs entries for MIXER crtc")
Signed-off-by: Pei Xiao <xiaopei01@kylinos.cn>
Acked-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c28f0dcb6a4526721d83ba1f659bba30564d3d54.1732087094.git.xiaopei01@kylinos.cn


Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>