fbdev: make get_fb_unmapped_area depends of !MMU

Even if CONFIG_FB_PROVIDE_GET_FB_UNMAPPED_AREA flag is selected
do not compile and use get_fb_unmapped_area() if CONFIG_MMU is
also set. This will avoid mmap errors when compiling multi
architectures at same time.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Yannick Fertre <yannick.fertre@st.com>
Cc: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>