virtio-gpu-3d: Drop workaround for VIRTIO_GPU_CAPSET_VIRGL2 define

In commit a8bff79e we added a definition to hw/virtio/virtio-gpu.h
for VIRTIO_GPU_CAPSET_VIRGL2, as a workaround for it not yet being
in the Linux kernel headers. In commit 77d361b1 we updated our
kernel headers to a version which does define the macro, so we can
now remove our workaround.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180622173249.29963-1-peter.maydell@linaro.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>