drm/exynos: Fix address space warning in exynos_drm_buf.c

Fixes the following warning:
drivers/gpu/drm/exynos/exynos_drm_buf.c:66:29: warning:
incorrect type in assignment (different address spaces)
drivers/gpu/drm/exynos/exynos_drm_buf.c:66:29:
expected void [noderef] <asn:2>*kvaddr
drivers/gpu/drm/exynos/exynos_drm_buf.c:66:29: got void *

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>