drm/i915: check the power well inside haswell_get_pipe_config

This fixes "unclaimed register" messages when booting with eDP only
and i915.disable_power_well=1.

The error messages were caused by:

  commit 0e8ffe1b
  Author: Daniel Vetter <daniel.vetter@ffwll.ch>
  Date:   Thu Mar 28 10:42:00 2013 +0100
      drm/i915: add hw state readout/checking for pipe_config

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>