fbdev: fix check for fb_mmap's mmio availability

Commit fc9bbca8 (vm: convert fb_mmap to
vm_iomap_memory() helper) made fbmem.c use vm_iomap_memory, but also
accidentally removed the check for mmio's availability.

Add the check back.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>