OMAP: iovmm: no gap checking for fixed address

If some fixed da address is wanted to be mapped and the page
is freed but it is used as gap, the mapping will fail.
This patch is fixing that and olny keeps the gap for
not fixed address.

Signed-off-by: Fernando Guzman Lugo <x0095840@ti.com>
Acked-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>