drm/nouveau/kms/nv50-: Use macros for DP registers in nouveau_dp.c

No functional changes.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-5-lyude@redhat.com