drm/amd/display: disable SG displays on cyan skillfish

drm/amd/display: disable SG displays on cyan skillfish

These parts were mainly for compute workloads, but they have
a display that was available for the console.  These chips
should support SG display, but I don't know that the support
was ever validated on Linux so disable it by default. It can
still be enabled by setting sg_display=1 for those that
want to play with it.  These systems also generally had large
carve outs so SG display was less of a factor.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3356
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>