drm/i915: Don't set cursor pipe select bits on g4x+

G4x cursor control registers still allow us to write to the pipe select
bits even though cursors are supposed to be fixed to a specific pipe.
Bspec tells us that we should only ever write 0 to these bits. Let's
follow that recommendation. On ilk+ the bits become hardwired to 0.

Also looks like ICL repurposes these bits for some other use, so
we had better stop setting them to bogus values there.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180130203807.13721-1-ville.syrjala@linux.intel.com


Reviewed-by: Mika Kahola <mika.kahola@intel.com>