drm/kmb: annotate set_test_mode_src_osc_freq_target_{low, hi}_bits() with __maybe_unused

drm/kmb: annotate set_test_mode_src_osc_freq_target_{low, hi}_bits() with __maybe_unused

Building with clang and and W=1 leads to warning about unused
set_test_mode_src_osc_freq_target_low_bits() and
set_test_mode_src_osc_freq_target_hi_bits(). Fix by annotating them with
__maybe_unused.

See also commit 6863f5643dd7 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").

Acked-by: Anitha Chrisanthus <anitha.chrisanthus@intel.com>
Tested-by: Nathan Chancellor <nathan@kernel.org> # build
Link: https://patchwork.freedesktop.org/patch/msgid/29a83771edd9b85032095ed3ecc1e91a77229b90.1725962479.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>