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>