drm/amdgpu: add S/G display parameter

Some users have reported flickerng with S/G display.  We've
tried extensively to reproduce and debug the issue on a wide
variety of platform configurations (DRAM bandwidth, etc.) and
a variety of monitors, but so far have not been able to.  We
disabled S/G display on a number of platforms to address this
but that leads to failure to pin framebuffers errors and
blank displays when there is memory pressure or no displays
at all on systems with limited carveout (e.g., Chromebooks).
Add a option to disable this as a debugging option as a
way for users to disable this, depending on their use case,
and for us to help debug this further.

v2: fix typo

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>