drm/i915/bxt: Don't save/restore eDP panel power during suspend (v3)

drm/i915/bxt: Don't save/restore eDP panel power during suspend (v3)

Our attempts save/restore panel power state in i915_suspend.c are
causing unclaimed register warnings on BXT since the registers for this
platform differ from older platforms.

The big hammer suspend/resume shouldn't be necessary for PP since the
connector/encoder hooks should already handle this.  In theory we could
remove this for all platforms, but in practice it's likely that would
cause some regressions since older platforms with LVDS may have
incomplete PP handling.  For now we'll leave the PCH save/restore alone
and change the non-PCH branch to only operate on gen <= 4 so that BXT
and future platforms aren't included.

v2: Typo fix: s/||/&&/

v3: Change non-PCH condition to a gen <= 4 test rather than listing
    VLV/CHV/BXT as specific platforms to exclude; should be more
    future-proof as we add new platforms.  (Daniel)

Cc: Vandana Kannan <vandana.kannan@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: drm-intel-fixes@lists.freedesktop.org
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452102821-17190-1-git-send-email-matthew.d.roper@intel.com
(cherry picked from commit e1ea07542352be468e901173c7a1beeee404d696)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>