drm/doc: Add function reference documentation for drm_mm.c

While at it do a tiny bit of interface cleanup and convert boolean
return values to bool. With this patch all exported functions and inline
helpers which are part of the drm_mm public interface are documented.

Also drop superflous extern function modifiers since most of drm_mm.h
doesn't use them - more consistent that way.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>