drm/i915/display: Remove kstrdup_const() and kfree_const() usage

drm/i915/display: Remove kstrdup_const() and kfree_const() usage

kstrdup_const() and kfree_const() can be confusing in code built as a
module. In such a case, it does not do what one could expect from the name
of the functions.

The code is not wrong by itself, but in such a case, it is equivalent to
kstrdup() and kfree().

So, keep thinks simple and straightforward.

This reverts commit 379b63e7e682 ("drm/i915/display: Save a few bytes of
memory in intel_backlight_device_register()")

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/f82be2ee3ac7d18dd9982b5368a88a5bf2aeb777.1727977199.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jani Nikula <jani.nikula@intel.com>