drm/amd/display/dc/core/dc_link_dp: Staticify local function 'linkRateInKHzToLinkRateMultiplier'

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

 drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dp.c:3710:19: warning: no previous prototype for ‘linkRateInKHzToLinkRateMultiplier’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dp.c: In function ‘dpcd_set_source_specific_data’:
 drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dp.c:4403:18: warning: variable ‘result_write_min_hblank’ set but not used [-Wunused-but-set-variable]

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
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>

drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c

@@ -3707,7 +3707,7 @@ bool detect_dp_sink_caps(struct dc_link *link)
 	/* TODO save sink caps in link->sink */
 }
 
-enum dc_link_rate linkRateInKHzToLinkRateMultiplier(uint32_t link_rate_in_khz)
+static enum dc_link_rate linkRateInKHzToLinkRateMultiplier(uint32_t link_rate_in_khz)
 {
 	enum dc_link_rate link_rate;
 	// LinkRate is normally stored as a multiplier of 0.27 Gbps per lane. Do the translation.