amdkfd: Memory availability can never be negative

Our assumptions about how much KFD memory is currently available for
allocation may be violated by various complexities so we define the
reported value as advisory, however we should never report negative
availability.

Signed-off-by: Daniel Phillips <daniel.phillips@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>