drm/modes: Make the whitelist more const

drm/modes: Make the whitelist more const

The commit 3764137906a5 ("drm/modes: Introduce a whitelist for the named
modes") introduced a whitelist in the named modes lookup code in order to
be a bit more robust.

However, even though the char pointers were made const, the array itself
was not. Let's fix that.

Fixes: 3764137906a5 ("drm/modes: Introduce a whitelist for the named modes")
Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190906074614.30608-1-mripard@kernel.org