drm/amdgpu/ras: use GPU PAGE_SIZE/SHIFT for reserving pages

We are reserving vram pages so they should be aligned to the
GPU page size.

Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>