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

In the Linux kernel, the following vulnerability has been resolved:

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.