drm/amd/display/dc/dce/dce_opp: Make local functions and ones invoked by reference static

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

 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:427:6: warning: no previous prototype for ‘dce60_opp_set_clamping’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:548:6: warning: no previous prototype for ‘dce60_opp_program_bit_depth_reduction’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:571:6: warning: no previous prototype for ‘dce60_opp_program_clamping_and_pixel_encoding’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:681:6: warning: no previous prototype for ‘dce60_opp_program_fmt’ [-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_opp.c

@@ -424,7 +424,7 @@ void dce110_opp_set_clamping(
  *		7 for programable
  *	2) Enable clamp if Limited range requested
  */
-void dce60_opp_set_clamping(
+static void dce60_opp_set_clamping(
 	struct dce110_opp *opp110,
 	const struct clamping_and_pixel_encoding_params *params)
 {
@@ -545,7 +545,7 @@ void dce110_opp_program_bit_depth_reduction(
 }
 
 #if defined(CONFIG_DRM_AMD_DC_SI)
-void dce60_opp_program_bit_depth_reduction(
+static void dce60_opp_program_bit_depth_reduction(
 	struct output_pixel_processor *opp,
 	const struct bit_depth_reduction_params *params)
 {
@@ -568,7 +568,7 @@ void dce110_opp_program_clamping_and_pixel_encoding(
 }
 
 #if defined(CONFIG_DRM_AMD_DC_SI)
-void dce60_opp_program_clamping_and_pixel_encoding(
+static void dce60_opp_program_clamping_and_pixel_encoding(
 	struct output_pixel_processor *opp,
 	const struct clamping_and_pixel_encoding_params *params)
 {
@@ -678,7 +678,7 @@ void dce110_opp_program_fmt(
 }
 
 #if defined(CONFIG_DRM_AMD_DC_SI)
-void dce60_opp_program_fmt(
+static void dce60_opp_program_fmt(
 	struct output_pixel_processor *opp,
 	struct bit_depth_reduction_params *fmt_bit_depth,
 	struct clamping_and_pixel_encoding_params *clamping)