drm/i915: check PPS regs for sanity when using eDP

If these regs don't have valid values, the panel won't come up, and may
even cause a system hang.  So do a basic sanity check when an eDP panel
is detected.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44305
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>