drm/i915/fbc: don't store/check a pointer to the FB

We already make sure we run intel_fbc_update_update during modesets
and page flips, and this function takes care of deactivating FBC, so
it shouldn't be possible for us to reach the condition we check at
intel_fbc_work_fn. So instead of grabbing framebuffer references and
adding a lot of code to track when we need to free them, just don't
track anything at all since we shouldn't need to.

v2: Rebase.
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453210558-7875-25-git-send-email-paulo.r.zanoni@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -961,7 +961,6 @@ struct intel_fbc {
 		bool scheduled;
 		u32 scheduled_vblank;
 		struct work_struct work;
-		struct drm_framebuffer *fb;
 	} work;
 
 	const char *no_fbc_reason;

drivers/gpu/drm/i915/intel_fbc.c

@@ -405,7 +405,6 @@ static void intel_fbc_work_fn(struct work_struct *__work)
 		goto retry;
 	}
 
-	if (crtc->base.primary->fb == work->fb)
 	fbc->activate(dev_priv);
 
 	work->scheduled = false;
@@ -441,7 +440,6 @@ static void intel_fbc_schedule_activation(struct intel_crtc *crtc)
 	 * we're not releasing fbc.lock, so it won't have an opportunity to grab
 	 * it to discover that it was cancelled. So we just update the expected
 	 * jiffy count. */
-	work->fb = crtc->base.primary->fb;
 	work->scheduled = true;
 	work->scheduled_vblank = drm_crtc_vblank_count(&crtc->base);
 	drm_crtc_vblank_put(&crtc->base);