omapfb: fix argument of blank operation

The blank operation should receive FB_BLANK_POWERDOWN, not VESA_POWERDOWN.

Signed-off-by: Felipe Contreras <felipe.contreras@nokia.com>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Andrea Righi <righi.andrea@gmail.com>
Acked-by: Trilok Soni <soni.trilok@gmail.com>
Cc: Imre Deak <imre.deak@solidboot.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>