drm/radeon: partially revert "fix VM_CONTEXT*_PAGE_TABLE_END_ADDR handling"

We have that bug for years and some users report side effects when fixing it on older hardware.

So revert it for VM_CONTEXT0_PAGE_TABLE_END_ADDR, but keep it for VM 1-15.

Signed-off-by: Christian König <christian.koenig@amd.com>
CC: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>