drm/amdgpu: Fix memory leak

drm_gem_object_put() should be paired with drm_gem_object_lookup().

All gem objs are saved in fb->base.obj[]. Need put the old first before
assign a new obj.

Trigger VRAM leak by running command below
$ service gdm restart

Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>