drm/radeon/cayman: set VM max pfn at MC init

No need to emit them at VM flush as we no longer use
variable sized page tables now that we support 2 level
page tables.  This matches the behavior of SI (which
does not support variable sized page tables).

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