drm/amdgpu: Explicitly add a flexible array at the end of 'struct amdgpu_bo_list'

'struct amdgpu_bo_list' is really used as if it was ended by a flex array.
So make it more explicit and add a 'struct amdgpu_bo_list_entry entries[]'
field at the end of the structure.

This way, struct_size() can be used when it is allocated.
It is less verbose.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>