drm/amdgpu: fix the page fault of raven2

drm/amdgpu: fix the page fault of raven2

While the apg_end address is 0xffffffff, if add 1 with it, the value will be
overflow and roll back to 0. So when 0 is written to
mmMC_VM_SYSTEM_APERTURE_HIGH_ADDR, the system aperture is actually disabled. And
so any access to vram will trigger a page fault.

Raven2's HW issue only need increase the vram end address, and needn't do it on
the agp.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>