drm/exynos: simplify the return value handling in exynos_dp_get_modes()

Just use 0 and 1 directly, instead of the confusing local variable
that's always set to 0.

Cc: Inki Dae <inki.dae@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/64cc680f14d961cedb726a6fd5c6dfd53ca9bb85.1709913674.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>