drm/nouveau/kms/nv50-: Remove open-coded drm_dp_read_desc()

Noticed this while going through our DP code - we use an open-coded
version of drm_dp_read_desc() instead of just using the helper, so
change that. This will also let us use quirks in the future if we end up
needing them.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-3-lyude@redhat.com