drm/amdgpu: stop splitting PTE commands into smaller ones

It doesn't make much sense to create bigger commands first which we then need
to split into smaller one again. Just make sure the commands we create aren't
to big in the first place.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>