drm/amdgpu/gmc6: disable legacy vga features in gmc init (v2)

Needs to be done when the MC is set up.

v2: make consistent with other asics

Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>