drm/i915: use the correct obj when preparing the sprite plane

Commit "drm/i915: create a prepare phase for sprite plane updates"
changed the old_obj pointer we use when committing sprite planes,
which caused a WARN() and a BUG() to be triggered. Later, commit
"drm/i915: use intel_fb_obj() macros to assign gem objects" introduced
the same problem to function intel_commit_sprite_plane().

Regression introduced by:
    commit ec82cb793c9224e0692eed904f43490cf70e8258
    Author: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
    Date:   Fri Oct 24 14:51:32 2014 +0100
        drm/i915: create a prepare phase for sprite plane updates
and:
    commit 77cde952
    Author: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
    Date:   Fri Oct 24 14:51:33 2014 +0100
        drm/i915: use intel_fb_obj() macros to assign gem objects

Credits to Imre Deak for pointing out the exact lines that were wrong.

v2: Also fix intel_commit_sprite_plane() (Ville)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85634


Testcase: igt/pm_rpm/legacy-planes
Testcase: igt/pm_rpm/legacy-planes-dpms
Testcase: igt/pm_rpm/universal-planes
Testcase: igt/pm_rpm/universal-planes-dpms
Credits-to: Imre Deak <imre.deak@intel.com>
Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>