drm/amdgpu: add missing irq.h include

BugLink: http://bugs.launchpad.net/bugs/1546572

this fixes the build on arm.
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit e9c5e740)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_irq.c

@@ -25,6 +25,7 @@
  *          Alex Deucher
  *          Jerome Glisse
  */
+#include <linux/irq.h>
 #include <drm/drmP.h>
 #include <drm/drm_crtc_helper.h>
 #include <drm/amdgpu_drm.h>