drm/i915: make sure eDP panel is turned on

When enabling the eDP port, we need to make sure the panel is turned on
after training the link.  If we don't, it likely won't come back after
suspend or may not come up at all.

For unknown reasons, unlocking the panel regs before initiating a power
on sequence is necessary.  There are known bugs in the PCH panel
sequencing logic, apparently this is one possible workaround.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=28739

.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: "Paulo J. S. Silva" <pjssilva@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>