drm/exynos: fix a potential error pointer dereference

Smatch reports the warning below:
drivers/gpu/drm/exynos/exynos_hdmi.c:1864 hdmi_bind()
error: 'crtc' dereferencing possible ERR_PTR()

The return value of exynos_drm_crtc_get_by_type maybe ERR_PTR(-ENODEV),
which can not be used directly. Fix this by checking the return value
before using it.

Signed-off-by: Xiang Yang <xiangyang3@huawei.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>