drm: export tile-group functions

Normally these are called from within drm core, from the EDID parsing
code.  But for dual-dsi in some drivers (at least drm/msm) we need to
call these from the driver.  So they should be exported.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Acked-by: Dave Airlie <airlied@redhat.com>