drm/arc: Stop consulting plane->fb

We want to stop using plane->fb with atomic driver, so stop looking at
it.

I have no idea what this code is trying to achieve. There is no
corresponding check in the enable path. Also since
arc_pgu_set_pxl_fmt() will anyway oops if there is no fb I'm going
to assuming that I can just remove the check entirely. There seems
to be a general shortage of .atomic_check() in this driver...

Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180405195035.24722-1-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Alexey Brodkin <abrodkin@synopys.com>

drivers/gpu/drm/arc/arcpgu_crtc.c

@@ -136,9 +136,6 @@ static void arc_pgu_crtc_atomic_disable(struct drm_crtc *crtc,
 {
 	struct arcpgu_drm_private *arcpgu = crtc_to_arcpgu_priv(crtc);
 
-	if (!crtc->primary->fb)
-		return;
-
 	clk_disable_unprepare(arcpgu->clk);
 	arc_pgu_write(arcpgu, ARCPGU_REG_CTRL,
 			      arc_pgu_read(arcpgu, ARCPGU_REG_CTRL) &