OMAP: DSS2: check lock_fb_info() return value

Give up if lock_fb_info() fails, following the same convention as other
lock_fb_info() users.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>