amdgpu: Unlock mutex if base_required is invalid

In the function amdgpu_vamgr_find_va() the function would return
without unlocking the mutex if the base_required offset was below
the va managers base offset.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>