drm/amdgpu/gmc: set a default disable value for AGP

To disable AGP, the start needs to be set to a higher
value than the end.  Set a default disable value for
the AGP aperture and allow the IP specific GMC code
to enable it selectively be calling amdgpu_gmc_agp_location().

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