drm/i915: Disable FBC on fastset if necessary, v2.

Without this, we will get a dmesg-warn when enable_fbc is cleared on a fastset:
WARN_ON(!crtc_state->enable_fbc)
WARNING: CPU: 0 PID: 1090 at drivers/gpu/drm/i915/intel_fbc.c:1091 intel_fbc_enable+0x2ce/0x580 [i915]
RIP: 0010:intel_fbc_enable+0x2ce/0x580 [i915]
Call Trace:
 ? __mutex_unlock_slowpath+0x46/0x2b0
 intel_update_crtc+0x6f/0x2b0 [i915]
 skl_update_crtcs+0x1d1/0x2b0 [i915]
 intel_atomic_commit_tail+0x1ea/0xdb0 [i915]
 intel_atomic_commit+0x244/0x330 [i915]
 drm_mode_atomic_ioctl+0x85d/0x950
 ? drm_atomic_set_property+0x970/0x970
 drm_ioctl_kernel+0x81/0xf0
 drm_ioctl+0x2de/0x390
 ? drm_atomic_set_property+0x970/0x970
 ? __handle_mm_fault+0x81b/0xfc0
 do_vfs_ioctl+0xa0/0x6e0
 ? __do_page_fault+0x2a5/0x550
 ksys_ioctl+0x35/0x60
 __x64_sys_ioctl+0x11/0x20
 do_syscall_64+0x55/0x190
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Changes since v1:
- Move intel_fbc_disable to intel_update_crtc() (Hans)

Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181220151719.30586-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>

drivers/gpu/drm/i915/intel_display.c

@@ -12752,7 +12752,9 @@ static void intel_update_crtc(struct drm_crtc *crtc,
 				       pipe_config);
 	}
 
-	if (new_plane_state)
+	if (pipe_config->update_pipe && !pipe_config->enable_fbc)
+		intel_fbc_disable(intel_crtc);
+	else if (new_plane_state)
 		intel_fbc_enable(intel_crtc, pipe_config, new_plane_state);
 
 	intel_begin_crtc_commit(crtc, old_crtc_state);

drivers/gpu/drm/i915/intel_fbc.c

@@ -1129,8 +1129,6 @@ void intel_fbc_disable(struct intel_crtc *crtc)
 	if (!fbc_supported(dev_priv))
 		return;
 
-	WARN_ON(crtc->active);
-
 	mutex_lock(&fbc->lock);
 	if (fbc->crtc == crtc)
 		__intel_fbc_disable(dev_priv);