drm/amdkfd: replace trivial funcs with direct access

These get funcs simply return an adev field. Replace funcs/calls with
direct field accesses instead.

Signed-off-by: Graham Sider <Graham.Sider@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>