drm/mgag200: make a const array static, makes object smaller

Don't populate the const array m_div_val on the stack but instead make
it static. Makes the object code smaller by 29 bytes:

Before:
   text	   data	  bss   dec    hex filename
  34736	   4552	    0 39288   9978 drivers/gpu/drm/mgag200/mgag200_mode.o

After:
   text	   data	  bss   dec    hex filename
  34625	   4616	    0 39241   9949 drivers/gpu/drm/mgag200/mgag200_mode.o

(gcc version 10.2.0)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210204191156.110778-1-colin.king@canonical.com