drm: sti: fix static checker warning in sti_awg_utils

The shift and the mask done on arg value is useless
since arg is null.

Signed-off-by: Vincent Abriou <[e-mail]>

drivers/gpu/drm/sti/sti_awg_utils.c

···
 				 * pixel. So we transform SKIP into SET
 				 * instruction */
 				opcode = SET;
-				arg = (arg << 24) >> 24;
-				arg &= (0x0ff);
 				break;
 			}