mcde: fix 'forbids mixed declarations and code' warning

  CC      drivers/video/mcde/dsilink_v2.o
drivers/video/mcde/dsilink_v2.c: In function 'enable_video_mode':
drivers/video/mcde/dsilink_v2.c:506:3: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]