drm/amd/display: set should_disable_otg storage-class-specifier to static

smatch reports
drivers/gpu/drm/amd/amdgpu/../display/dc/clk_mgr/dcn315/dcn315_clk_mgr.c:90:6:
  warning: symbol 'should_disable_otg' was not declared. Should it be static?

should_disable_otg() is only used in dcn315_clk_mgr.c, so it should be static
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/clk_mgr/dcn315/dcn315_clk_mgr.c

@@ -87,7 +87,7 @@ static int dcn315_get_active_display_cnt_wa(
 	return display_count;
 }
 
-bool should_disable_otg(struct pipe_ctx *pipe)
+static bool should_disable_otg(struct pipe_ctx *pipe)
 {
 	bool ret = true;