drm/exynos: do not return negative values from .get_modes()

drm/exynos: do not return negative values from .get_modes()

The .get_modes() hooks aren't supposed to return negative error
codes. Return 0 for no modes, whatever the reason.

Cc: Inki Dae <inki.dae@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: stable@vger.kernel.org
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/d8665f620d9c252aa7d5a4811ff6b16e773903a2.1709913674.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>