drm/panthor: Explicitly include mm.h for the {virt, __phys)_to_pfn() defs

drm/panthor: Explicitly include mm.h for the {virt, __phys)_to_pfn() defs

Something on arm[64] must be including <asm/page.h>, but things fail
to compile on sparc64. Make sure this header is included (through
linux/mm.h) so this driver can be compile-tested on all supported
architectures.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202403031142.Vl4pW7X6-lkp@intel.com/
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240304090812.3941084-3-boris.brezillon@collabora.com