[kernel] Pass the fb phys addr to userspace

Instead of always mapping the framebuffer at an arbitrary location, and
so reporting that to userspace, send the physical address so drivers can
call system_map_mmio().

src/kernel/scheduler.cpp

@@ -28,8 +28,6 @@
 
 scheduler *scheduler::s_instance = nullptr;
 
-extern kernel::args::framebuffer *fb;
-
 const uint64_t rflags_noint = 0x002;
 const uint64_t rflags_int = 0x202;