drm/amdgpu: disable vga render in dce hw_init

This got dropped accidently with the fb location changes, but for
some reason, this doesn't seem to cause an issue on all cards which
is why I never saw it despite extensive testing.  I suspect it may
only be an issue on systems with a legacy sbios that enables vga.

Tested-by: Andres Rodriguez <andresx7@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>