drm/exynos: dsi: use IS_ERR() to check devm_ioremap_resource() results

devm_ioremap_resource() returns an error pointer, not NULL. Thus,
the result should be checked with IS_ERR().

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --cc drivers/gpu/drm/exynos/exynos_drm_dsi.c
Simple merge