hw/display/milkymist-tmu2: Move inlined code from header to source

hw/display/milkymist-tmu2: Move inlined code from header to source

Move the complexity of milkymist_tmu2_create() into the
source file. Doing so we avoid to include the X11/OpenGL
headers in all LM32 devices, and we also avoid the duplicate
declaration of glx_fbconfig_attr[] (it is already declared
in hw/display/milkymist-tmu2.c).
Since TYPE_MILKYMIST_TMU2 is now accessible, use it.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190130120005.23123-5-philmd@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>