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 <[e-mail]>
Tested-by: Nathan Chancellor <[e-mail]> # build
Link: https://patchwork.freedesktop.org/patch/msgid/29a[e-mail]
Signed-off-by: Jani Nikula <[e-mail]>

drivers/gpu/drm/kmb/kmb_dsi.c

···
 	}
 }
 
-static inline void
+static inline __maybe_unused void
 	set_test_mode_src_osc_freq_target_low_bits(struct kmb_dsi *kmb_dsi,
 						   u32 dphy_no,
 						   u32 freq)
···
 		       (freq & 0x7f));
 }
 
-static inline void
+static inline __maybe_unused void
 	set_test_mode_src_osc_freq_target_hi_bits(struct kmb_dsi *kmb_dsi,
 						  u32 dphy_no,
 						  u32 freq)