omapfb: Blizzard: fix pointer to be const

Fixes a compiler warning:
warning: assignment discards qualifiers from pointer target type

Cc: Tomi Valkeinen <tomi.valkeinen@nokia.com>
Cc: Imre Deak <imre.deak@nokia.com>
Signed-off-by: Tommi Rantala <tt.rantala@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>