drm/amdgpu/gmc: disable keep_stolen_vga_memory on arcturus

I suspect the only reason this was set was to avoid touching
the display related registers on arcturus.  Someone should
double check this on arcturus with S3.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>