drm/i915: skip FDI & PCH enabling for DP_A

eDP on the CPU doesn't need the PCH set up at all, it can in fact cause
problems.  So avoid FDI training and PCH PLL enabling in that case.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>