drm/amdgpu: Use GEM obj reference for KFD BOs

Releasing the AMDGPU BO ref directly leads to problems when BOs were
exported as DMA bufs. Releasing the GEM reference makes sure that the
AMDGPU/TTM BO is not freed too early.

Also take a GEM reference when importing BOs from DMABufs to keep
references to imported BOs balances properly.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Tested-by: Alex Sierra <alex.sierra@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Sierra <alex.sierra@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>