drm/amdgpu: Drop eviction lock when allocating PT BO

Re-take the eviction lock immediately again after the allocation is
completed, to fix circular locking warning with drm_buddy allocator.

Move amdgpu_vm_eviction_lock/unlock/trylock to amdgpu_vm.h as they are
called from multiple files.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>