drm/cma: Fix recent regression of mmap() in the MMU case.

The stub get_unmapped_area() function was actually getting called, so
all of our mmap()s failed.

Cc: Yannick Fertre <yannick.fertre@st.com>
Fixes: 97bf3a9a

 ("drm/cma: Update DEFINE_DRM_GEM_CMA_FOPS to add get_unmapped_area")
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170417233124.18420-1-eric@anholt.net
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>