drm/i915: disable the cpu edp port after the cpu pipe

See bspec, Vol3 Part2, Section 1.1.3 "Display Mode Set Sequence". This
applies to all platforms where we currently support eDP on, i.e. ilk,
snb & ivb.

Without this change we fail to light up the eDP port on previously
unused crtcs (likely because something is stuck on the old pipe), and
we also fail to properly disable the old pipe (i.e. bit 30 in the
PIPECONF register is stuck as set until the next reboot).

v2: Rebased on top of the edp panel off sequence changes in 3.6-rc2.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=44001


Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>