drm: mxsfb: Fix NULL pointer dereference crash on unload

The mxsfb->crtc.funcs may already be NULL when unloading the driver,
in which case calling mxsfb_irq_disable() via drm_irq_uninstall() from
mxsfb_unload() leads to NULL pointer dereference.

Since all we care about is masking the IRQ and mxsfb->base is still
valid, just use that to clear and mask the IRQ.

Fixes: ae1ed009 ("drm: mxsfb: Stop using DRM simple display pipeline helper")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Abrecht <public@danielabrecht.ch>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Stefan Agner <stefan@agner.ch>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211016210446.171616-1-marex@denx.deSigned-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/mxsfb/mxsfb_drv.c

@@ -173,7 +173,11 @@ static void mxsfb_irq_disable(struct drm_device *drm)
 	struct mxsfb_drm_private *mxsfb = drm->dev_private;
 
 	mxsfb_enable_axi_clk(mxsfb);
-	mxsfb->crtc.funcs->disable_vblank(&mxsfb->crtc);
+
+	/* Disable and clear VBLANK IRQ */
+	writel(CTRL1_CUR_FRAME_DONE_IRQ_EN, mxsfb->base + LCDC_CTRL1 + REG_CLR);
+	writel(CTRL1_CUR_FRAME_DONE_IRQ, mxsfb->base + LCDC_CTRL1 + REG_CLR);
+
 	mxsfb_disable_axi_clk(mxsfb);
 }