drm/i915/ibx, cpt: Don't attempt to register eDP if LVDS was detected

Atm on IBX/CPT we attempt to detect if eDP is present even if LVDS was
already detected and an encoder for it was registered. This involves
trying to read out the eDP DPCD, which in turn needs the same power
sequencer that LVDS uses. Poking at the VDD line at an unexpected time
may or may not interfere with the LVDS panel, but it's probably safer to
prevent this. Registering both an LVDS and an eDP connector would also
present a similar problem accessing the shared PPS at any point later in
an unexpected way.

We also need this to be able fix PPS initialization before its first use
in the next patch. For that we want to be sure that PPS is not in use
by LVDS.

v2:
- Split out the PPS init fix to a separate patch. (Chris)
- Add comment about eDP init depending on LVDS init. (Chris)
- Make the use of the intel_encoder ptr less error prone.
v3:
- Use IBX/CPT reference instead of the incorrect ILK, add a WARN about
  this. (Ville)
v4:
- Use a helper to get the lvds encoder instead of opencoding the same.
  (Ville)

CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v2)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v3)
Link: http://patchwork.freedesktop.org/patch/msgid/1466499109-20240-2-git-send-email-imre.deak@intel.com