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

3 changes:
- If a given value is unset, use the maximal limits from the eDP spec.
- Write back the new values, since otherwise the panel power sequencing
  hw will not dtrt.
- Revert the early bail-out in case the register values are unset.

The last change reverts

commit bfa3384a
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Tue Apr 10 11:58:04 2012 -0700

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

v2:
- Unlock the PP regs as the very first thing. This is a required w/a
  for cpu eDP on port A, and generally a good idea.
- Fixup the panel power control port selection bits.

v3: Paulo Zanoni noticed that I've fumbled the computation of the spec
limit values. Fix them up. We've also noticed that the t8/t9 values in
the vbt/bios-programmed pp are much larger than any limits. My guess
is that this is to conceal any backlight enable/disable delays. So by
using the much shorter limits from the spec, which only concerns the
sink, we risk that we might display before the backlight is fully on,
or disable the output while the backlight still has afterglow. I've
figured I don't care too much, since this will only happen when both
the pp regs are not programmed, and the vbt tables don't contain
anything useful.

v4: Don't set the port selection bits on hsw/LPT, they don't exist any
more.

v5: Fixup spelling issues in comments, as noticed by Jesse Barnes.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>