drm/amd/amdgpu: fix refcount leak

[Why]
the gem object rfb->base.obj[0] is get according to num_planes
in amdgpufb_create, but is not put according to num_planes

[How]
put rfb->base.obj[0] in amdgpu_fbdev_destroy according to num_planes

Signed-off-by: Jingwen Chen <Jingwen.Chen2@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>