The current values seem to be defined in a format that's specific to the
i915, gma500 and radeon drivers. To make this more generally useful, use
the values as defined in the specification.

While at it, prefix the constants with DP_ for improved namespacing.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>