vgaarb: mark vga_tryget static

This symbols isn't used anywhere outside of vgaarb.c.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200801061713.307434-1-hch@lst.de