drm/nouveau/kms: Start using drm_dp_read_dpcd_caps()

Now that we've extracted i915's code for reading both the normal DPCD
caps and extended DPCD caps into a shared helper, let's start using this
in nouveau to enable us to start checking extended DPCD caps for free.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-21-lyude@redhat.com