drm/i915: In intel_dp_init, replace read of DPCD with intel_dp_get_dpcd

Eliminates an open-coded read and also gains the retry behaviour of
intel_dp_get_dpcd, which seems like a good idea.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Adam Jackson <ajax@redhat.com>