Future hardware generations can handle PRT flags on a per page basis,
but current hardware can only turn it on globally.

Add the basic handling for both, a global callback to enable/disable
triggered by setting a per mapping flag.

v2: agd: rebase fixes
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>