exynos: fix type-punned pointer build warning

As like the commit ecc2a097 ("xf86drm: Fix type-punned pointer
build warning"), this fixes following build warning.

   exynos_drm.c: In function 'exynos_handle_event':
   exynos_drm.c:420:15: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
      e = (struct drm_event *) &buffer[i];
                  ^

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Reviewed-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>