drm/i915: Enable display workaround 827 for all planes, v2.

The workaround was applied only to the primary plane, but is required
on all planes. Iterate over all planes in the crtc atomic check to see
if the workaround is enabled, and only perform the actual toggling in
the pre/post plane update functions.

Changes since v1:
- Track active NV12 planes in a nv12_planes bitmask. (Ville)

v2: Removing BROXTON support for NV12 due to WA826

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1526074397-10457-2-git-send-email-vidya.srinivas@intel.com