drm: exynos: include linux/irq.h

I ran into a build error on x86:

drivers/gpu/drm/exynos/exynos5433_drm_decon.c: In function 'decon_conf_irq':
drivers/gpu/drm/exynos/exynos5433_drm_decon.c:706:2: error: implicit declaration of function 'irq_set_status_flags'; did you mean 'dquot_state_flag'? [-Werror=implicit-function-declaration]
  irq_set_status_flags(irq, IRQ_NOAUTOEN);

Adding the missing include fixes the error.

Fixes: b37d53a0

 ("drm/exynos/decon5433: move TE handling to DECON")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>