drm/amd/display/dc/dce/dce_link_encoder: Make functions invoked by reference static

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

 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_link_encoder.c:1200:6: warning: no previous prototype for ‘dce60_link_encoder_enable_dp_output’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_link_encoder.c:1239:6: warning: no previous prototype for ‘dce60_link_encoder_enable_dp_mst_output’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_link_encoder.c:1429:6: warning: no previous prototype for ‘dce60_link_encoder_dp_set_phy_pattern’ [-Wmissing-prototypes]

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: Mauro Rossi <issor.oruam@gmail.com>
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/dce/dce_link_encoder.c

@@ -1197,7 +1197,7 @@ void dce110_link_encoder_enable_dp_mst_output(
 
 #if defined(CONFIG_DRM_AMD_DC_SI)
 /* enables DP PHY output */
-void dce60_link_encoder_enable_dp_output(
+static void dce60_link_encoder_enable_dp_output(
 	struct link_encoder *enc,
 	const struct dc_link_settings *link_settings,
 	enum clock_source_id clock_source)
@@ -1236,7 +1236,7 @@ void dce60_link_encoder_enable_dp_output(
 }
 
 /* enables DP PHY output in MST mode */
-void dce60_link_encoder_enable_dp_mst_output(
+static void dce60_link_encoder_enable_dp_mst_output(
 	struct link_encoder *enc,
 	const struct dc_link_settings *link_settings,
 	enum clock_source_id clock_source)
@@ -1426,7 +1426,7 @@ void dce110_link_encoder_dp_set_phy_pattern(
 
 #if defined(CONFIG_DRM_AMD_DC_SI)
 /* set DP PHY test and training patterns */
-void dce60_link_encoder_dp_set_phy_pattern(
+static void dce60_link_encoder_dp_set_phy_pattern(
 	struct link_encoder *enc,
 	const struct encoder_set_dp_phy_pattern_param *param)
 {