drm/exynos: use of_get_named_gpio to get hdmi hpd gpio

Cleanup by removing flags variable from drm_hdmi_dt_parse_pdata
which is not used anywhere. Swtiching to of_get_named_gpio instead
of of_get_named_gpio_flags solved this.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Acked-by: Seung-Woo Kim <sw0312@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>