drm/i915: disable PSR by default on HSW/BDW

We've been ignoring the poor bugzilla reporters that say PSR causes
system lockups and all other sorts of problems. The earliest bug
report is from April, so I think we can use the "revert the offending
commit if no fixes are presented within 8 months" rule here.

Fixes: 9b58e352

 ("drm/i915: Enable PSR by default on Haswell and Broadwell.")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97602
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97515
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96736
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96704
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96569
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95176
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94985
Cc: <stable@vger.kernel.org> # v4.6+
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481662664-18986-1-git-send-email-paulo.r.zanoni@intel.com