arch: hexagon: include: asm: add "vga.h" in Kbuild

Need include generic "vga.h", or can not pass compiling with
allmodconfig, the related error:

    CC [M]  drivers/gpu/drm/drm_irq.o
  In file included from include/linux/vgaarb.h:34:0,
                   from drivers/gpu/drm/drm_irq.c:42:
  include/video/vga.h:22:21: fatal error: asm/vga.h: No such file or directory

Also move "preempt.h" upper to match sort order.
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>

arch/hexagon/include/asm/Kbuild

@@ -33,6 +33,7 @@ generic-y += pci.h
 generic-y += percpu.h
 generic-y += poll.h
 generic-y += posix_types.h
+generic-y += preempt.h
 generic-y += resource.h
 generic-y += rwsem.h
 generic-y += scatterlist.h
@@ -55,5 +56,5 @@ generic-y += trace_clock.h
 generic-y += types.h
 generic-y += ucontext.h
 generic-y += unaligned.h
+generic-y += vga.h
 generic-y += xor.h
-generic-y += preempt.h