omapfb: HWA742: fix pointer to be const

Fixes the following:
warning: assignment discards qualifiers from pointer target type

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>