drm/radeon: make array encoded_lanes static

Don't populate the read-only array encoded_lanes on the stack but instead it
static. Also makes the object code smaller by 97 bytes:

Before:
   text    data    bss     dec    hex filename
  38899    8064      0   46963   b773 ./drivers/gpu/drm/radeon/r600_dpm.o

After:
   text    data    bss     dec    hex filename
  38738    8128      0   46866   b712 ./drivers/gpu/drm/radeon/r600_dpm.o

(gcc version 11.2.0)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>