OMAP: DSS2: OMAPFB: Check var even if there isn't memory

If video memory hasn't been allocate have check_fb_var() still check
most of the settings, just skip the ones involving the size of the
memory region. Also skip the memory address calculations in
omapfb_setup_overlay() if there's no memory.

Signed-off-by: Ville Syrjälä <ville.syrjala@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>