fbmem: add a default get_fb_unmapped_area function

Allow generic frame-buffer to provide a default
get_fb_unmapped_area function if specific devices need it.

Usually this function is defined in architecture directories but
define it here may limit code duplication especially for all ARM
platforms without MMU.

version 5:
- set get_unmapped_area field if FB_PROVIDE_GET_FB_UNMAPPED_AREA is
  defined

version 4:
- introdude a configuration flag to be independent of architecture

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483521177-21794-2-git-send-email-benjamin.gaignard@linaro.org