drm/radeon: reduce number of free VMIDs and pipes in KV

To support HSA on KV, we need to limit the number of vmids and pipes
that are available for radeon's use with KV.

This patch reserves VMIDs 8-15 for amdkfd (so radeon can only use VMIDs
0-7) and also makes radeon thinks that KV has only a single MEC with a single
pipe in it

v3: Use define for static vmid allocation in radeon

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>