drm/i915: Don't reject primary plane windowing with color keying enabled on SKL+

On SKL+ plane scaling is mutually exclusive with color keying. The code
check for this, but during some refactoring the code got changed to
also reject primary plane windowing when color keying is used. There is
no such restriction in the hardware, so restore the original logic.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: 061e4b8d

 ("drm/i915: clean up atomic plane check functions, v2.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452883613-28549-1-git-send-email-ville.syrjala@linux.intel.com
Cc: stable@vger.kernel.org
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
(cherry picked from commit 693bdc28

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>