drm/i915: Remove pointless planes_changed=true assignment

i915 doesn't use the crtc_state->plane_changed flag for anything,
so setting it is pointless.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708125325.16576-4-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>