drm/amdkfd: remove kgd_dev declaration and initialization

Completes removal of kgd_dev. Direct references to amdgpu_device objects
should now be used 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>