drm/amd/display: add virtual_setup_stream_attribute decl to header

Smatch reports this issue
virtual_link_hwss.c:32:6: warning: symbol
  'virtual_setup_stream_attribute' was not declared.
  Should it be static?

virtual_setup_stream_attribute is only used in
virtual_link_hwss.c, but the other functions in the
file are declared in the header file and used elsewhere.
For consistency, add the virtual_setup_stream_attribute
decl to virtual_link_hwss.h.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>