drm/i915/eDP: do not write power sequence registers for ghost eDP

Some machines detect an eDP port even if it's not really there, and eDP
initialization has a fail path for this. Typically such machines have an
LVDS display instead. A regression introduced in

commit 82ed61fa
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sat Oct 20 20:57:41 2012 +0200

    drm/i915: make edp panel power sequence setup more robust

updated the power sequence registers PCH_PP_ON_DELAYS, PCH_PP_OFF_DELAYS,
and PCH_PP_DIVISOR also in the ghost eDP case, messing up the LVDS display.

Split the power sequencer initialization into two, delaying the register
updates until after we know the eDP is real.

Note: Keep the PP_CONTROL unlocking in the first part, even if it does not
update registers, per the commit message of the above mentioned commit.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=52601


Reported-and-tested-by: Ryan Coe <ryan@rycomotorsports.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>