drm/dp: Staticize a couple of DP utility functions

sparse complains about these functions missing a prototype, but looking
closer they aren't (and likely not supposed to be) used outside of this
source file, so they can be made static.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>