drm/i915: add missing error capturing of the PIPESTAT reg

While checking the error capture path I noticed that we lacked the
power domain-on check for PIPESTAT so fix this by moving that to where
the rest of pipe registers are captured.

The move also revealed that we actually don't include this register in
the error report, so fix that too.

v2:
- patch introduced in v2 of the patchset
v3:
- add back !HAS_PCH_SPLIT check (Ville)
[ Ignore my previous comment about the gen<=5 || vlv check, I realized
  that it's the same as !HAS_PCH_SPLIT. ]

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>