drm/nouveau/bios: make const arrays hwsq_signature and edid_sig static

Don't populate arrays hwsq_signature and edid_sig on the stack but
instead make them static. Makes the object code smaller by over 190
bytes:

Before:
   text    data     bss     dec     hex filename
  35676    3312      64   39052    988c nouveau_bios.o

After:
   text    data     bss     dec     hex filename
  35319    3472      64   38855    97c7 nouveau_bios.o

(gcc version 7.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>