drm/amd/display/dc/dc_hdmi_types: Move string definition to the only file it's used in

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

 drivers/gpu/drm/amd/amdgpu/../display/dc/dc_hdmi_types.h:53:22:
   warning: ‘dp_hdmi_dongle_signature_str’ defined but not used [-Wunused-const-variable=]

[snipped 400 similar lines for brevity]

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Wenjing Liu <wenjing.liu@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>