drm/i915: Don't call intel_update_fbc from intel_crtc_cursor_set

Commit 74dff282

 exposed this unnecessary call by causing a change in
the failure path on i965 where framebuffer compression will be turned
on and off on every cursor update. If you don't have the xf86-video-intel
fix to avoid the blinking cursor effect, this is very slow.

Symptoms were a far more noticeable cursor blink with every cursor image
change combined with severe slowdown for animated cursors.

Signed-off-by: Brian Rogers <brian@xyzw.org>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>