drm/amd/display/amdgpu_dm/amdgpu_dm_helpers: Move SYNAPTICS_DEVICE_ID into CONFIG_DRM...

drm/amd/display/amdgpu_dm/amdgpu_dm_helpers: Move SYNAPTICS_DEVICE_ID into CONFIG_DRM_AMD_DC_DCN ifdef

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

 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_helpers.c:48:22: warning: ‘SYNAPTICS_DEVICE_ID’ defined but not used [-Wunused-const-variable=]

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: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>