drm/amdgpu: Reserve 0-2 invalidation reg sets for none-amdgpu usages

Firmware used reg set 2 for tlb invalidation. AMDGPU can start from reg
set 3 to avoid the conflict. AMDKFD will use the reg set 0 or 1 when
necesary.

Signed-off-by: Shaoyun Liu <Shaoyun.Liu@amd.com>
Reviewws-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>