drm/i915: Ignore stale wm register values on resume on ilk-bdw (v2)

When we resume the watermark register may contain some BIOS leftovers,
or just the hardware reset values. We should ignore those as the
pipes will be off anyway, and so frobbing around with intermediate
watermarks doesn't make much sense.

In fact I think we should just throw the skip_intermediate_wm flag
out, and instead properly sanitize the "active" watermarks to match
the current plane and pipe states. The actual wm state readout might
also need a bit of work. But for now, let's continue with the
skip_intermediate_wm to keep the fix more minimal.

Fixes this sort of errors on resume
[drm:ilk_validate_pipe_wm] LP0 watermark invalid
[drm:intel_crtc_atomic_check] No valid intermediate pipe watermarks are possible
[drm:intel_display_resume [i915]] *ERROR* Restoring old state failed with -22
and a boatload of subsequent modeset BAT fails on my ILK.

v2:
 - Rebase; the SKL atomic WM patches that just landed changed the WM
   structure fields in intel_crtc_state slightly.  (Matt)

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: ed4a6a7c ("drm/i915: Add two-stage ILK-style watermark programming (v11)")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463159442-20478-1-git-send-email-matthew.d.roper@intel.com