ARM: pxafb: Fix access to nonexistent member of pxafb_info

In case CONFIG_FB_PXA_OVERLAY is not defined, the pxafb_freq_transition()
function tests nonexistent member of pxafb_info (since the member is not
part of the structure).

Fix this by wraping the test in ifdef, even if I don't really like how the code
looks now. The check doesn't have to happen if overlays are disabled at all as
the check is always true then.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>