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 77cde95217484e845743818691df026cec2534f4
    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>

drivers/gpu/drm/i915/intel_sprite.c

@@ -1264,10 +1264,11 @@ intel_prepare_sprite_plane(struct drm_plane *plane,
 	struct drm_device *dev = plane->dev;
 	struct drm_crtc *crtc = state->crtc;
 	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
+	struct intel_plane *intel_plane = to_intel_plane(plane);
 	enum pipe pipe = intel_crtc->pipe;
 	struct drm_framebuffer *fb = state->fb;
 	struct drm_i915_gem_object *obj = intel_fb_obj(fb);
-	struct drm_i915_gem_object *old_obj = intel_fb_obj(plane->fb);
+	struct drm_i915_gem_object *old_obj = intel_plane->obj;
 	int ret;
 
 	if (old_obj != obj) {
@@ -1302,7 +1303,7 @@ intel_commit_sprite_plane(struct drm_plane *plane,
 	enum pipe pipe = intel_crtc->pipe;
 	struct drm_framebuffer *fb = state->fb;
 	struct drm_i915_gem_object *obj = intel_fb_obj(fb);
-	struct drm_i915_gem_object *old_obj = intel_fb_obj(plane->fb);
+	struct drm_i915_gem_object *old_obj = intel_plane->obj;
 	int crtc_x, crtc_y;
 	unsigned int crtc_w, crtc_h;
 	uint32_t src_x, src_y, src_w, src_h;