drm/amdgpu: stop allocating a page array for prime shared BOs

We don't need the page array for prime shared BOs, stop allocating it.

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