drm/amd/display: Make some functions static

Fix the following sparse warnings:

drivers/gpu/drm/amd/amdgpu/../display/dc/dce120/dce120_resource.c:483:21: warning: symbol 'dce120_clock_source_create' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/dce120/dce120_resource.c:506:6: warning: symbol 'dce120_clock_source_destroy' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/dce120/dce120_resource.c:513:6: warning: symbol 'dce120_hw_sequencer_create' was not declared. Should it be static?

Fixes: b8fdfcc6 ("drm/amd/display: Add DCE12 core support")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai26@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce120/dce120_resource.c

@@ -480,7 +480,7 @@ static const struct dc_debug_options debug_defaults = {
 		.disable_clock_gate = true,
 };
 
-struct clock_source *dce120_clock_source_create(
+static struct clock_source *dce120_clock_source_create(
 	struct dc_context *ctx,
 	struct dc_bios *bios,
 	enum clock_source_id id,
@@ -503,14 +503,14 @@ struct clock_source *dce120_clock_source_create(
 	return NULL;
 }
 
-void dce120_clock_source_destroy(struct clock_source **clk_src)
+static void dce120_clock_source_destroy(struct clock_source **clk_src)
 {
 	kfree(TO_DCE110_CLK_SRC(*clk_src));
 	*clk_src = NULL;
 }
 
 
-bool dce120_hw_sequencer_create(struct dc *dc)
+static bool dce120_hw_sequencer_create(struct dc *dc)
 {
 	/* All registers used by dce11.2 match those in dce11 in offset and
 	 * structure