drm/i915: Don't try to use SPR_SCALE when we don't have a sprite scaler

Haswell does not have a scaler in the sprite pipeline anymore, so let's
ensure:
  1/ We bail out of update_plate() when someone is trying to ask to
     display a scaled framebuffer,
  2/ We never write to the nonexistent SPR_SCALE register

v2: Smash in the fixup from Damien in the disable_plane function.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> (for v1)
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org> (for v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 26d70ea..3b590ba 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -229,6 +229,7 @@
 	struct drm_plane base;
 	enum pipe pipe;
 	struct drm_i915_gem_object *obj;
+	bool can_scale;
 	int max_downscale;
 	u32 lut_r[1024], lut_g[1024], lut_b[1024];
 	void (*update_plane)(struct drm_plane *plane,