drm/dp-helper: Move the legacy helpers to gma500

Except for gma500 all drivers are converted to the new style helpers,
which have much better abstraction of the underlying hw protocols and
already much more helper functions (including the entire mst library)
on top of them. Since no one seems to work on converting gma500 let's
just move the code away so that new drivers don't end up accidentally
using this.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
[danvet: Add __deprecated as requested by Alan. Also add a short FIXME
comment and drop the EXPORT_SYMBOL which is no longer needed.]
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>