drm/amdgpu: Change GART offset to 64-bit

The GART aperture size can be bigger than 4GB. Therefore the offset
used in amdgpu_gart_bind and amdgpu_gart_unbind must be 64-bit.

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