drm/i915: Rename i915_dp_detect_common to intel_dp_get_dpcd

This describes the function better, allowing it to be used where the
DPCD value is relevant.

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