drm/i915: enable PCH PLL, FDI training and transcoder even for eDP

eDP panels require these to be set up prior to panel power sequencing,
or they'll fail to power on due to an "asset not ready" check.  And of
course, eDP panels attached to anything other than DP_A need them
enabled regardless, since they'll be driven from the CPU through FDI out
to the PCH.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>