drm/exynos: exynos7_drm_decon: add vblank check in IRQ handling

If there's support for another console device (such as a TTY serial),
the kernel occasionally panics during boot. The panic message and a
relevant snippet of the call stack is as follows:

  Unable to handle kernel NULL pointer dereference at virtual address 000000000000000
  Call trace:
    drm_crtc_handle_vblank+0x10/0x30 (P)
    decon_irq_handler+0x88/0xb4
    [...]

Otherwise, the panics don't happen. This indicates that it's some sort
of race condition.

Add a check to validate if the drm device can handle vblanks before
calling drm_crtc_handle_vblank() to avoid this.

Cc: stable@vger.kernel.org
Fixes: 96976c3 ("drm/exynos: Add DECON driver")
Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

Author: Kaustabh Chakraborty

drivers/gpu/drm/exynos/exynos7_drm_decon.c

@@ -636,6 +636,10 @@ static irqreturn_t decon_irq_handler(int irq, void *dev_id)
 	if (!ctx->drm_dev)
 		goto out;
 
+	/* check if crtc and vblank have been initialized properly */
+	if (!drm_dev_has_vblank(ctx->drm_dev))
+		goto out;
+
 	if (!ctx->i80_if) {
 		drm_crtc_handle_vblank(&ctx->crtc->base);