Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/si_dpm.c:3802:4: warning: no previous prototype for ‘si_get_ddr3_mclk_frequency_ratio’ [-Wmissing-prototypes]
 3802 | u8 si_get_ddr3_mclk_frequency_ratio(u32 memory_clock)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/si_dpm.c:3815:4: warning: no previous prototype for ‘si_get_mclk_frequency_ratio’ [-Wmissing-prototypes]
 3815 | u8 si_get_mclk_frequency_ratio(u32 memory_clock, bool strobe_mode)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/si_dpm.c:3897:6: warning: no previous prototype for ‘si_trim_voltage_table_to_fit_state_table’ [-Wmissing-prototypes]
 3897 | void si_trim_voltage_table_to_fit_state_table(struct radeon_device *rdev,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>