drm/i915: Wait for pending page flips before enabling/disabling the primary plane

We have to write to the primary plane base address registrer when we
enable/disable the primary plane in response to sprite coverage. Those
writes will cause the flip counter to increment which could interfere
with the detection of CS flip completion. We could end up completing
CS flips before the CS has even executed the commands from the ring.

To avoid such issues, wait for CS flips to finish before we toggle the
primary plane on/off.

v2: Rebased due to atomic sprite update changes

Testcase: igt/kms_mmio_vs_cs_flip/setplane_vs_cs_flip
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_drv.h

@@ -802,6 +802,8 @@ int valleyview_get_vco(struct drm_i915_private *dev_priv);
 void intel_mode_from_pipe_config(struct drm_display_mode *mode,
 				 struct intel_crtc_config *pipe_config);
 int intel_format_to_fourcc(int format);
+void intel_crtc_wait_for_pending_flips(struct drm_crtc *crtc);
+
 
 /* intel_dp.c */
 void intel_dp_init(struct drm_device *dev, int output_reg, enum port port);