drm/i915: pin sprite fb only if it changed

Optimize code avoiding helding dev mutex if old fb and current fb
are the same.

v2: take Ville's comments
	- move comment along with the pin_and_fence call
	- check for error before calling i915_gem_track_fb
	- move old_obj != obj to an upper if condition

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>