drm/amdkfd: add a new flag to manage where VRAM allocations go

On big and small APUs we send KFD VRAM allocations to GTT
since the carve out is either non-existent or relatively
small.  However, if someone sets the carve out size to be
relatively large, we may end up using GTT rather than VRAM.

No change of logic with this patch, but it allows the
driver to determine which logic to use based on the
carve out size in the future.

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>