drm/i915: read full receiver capability field during DP hot plug

Read link status first, followed by the full DPCD receiver cap field
rather than just the first 8 bytes.

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