drm/bochs: make structure bochs_bo_driver static

The structure bochs_bo_driver is local to the source and does not need to
be in global scope, so make it static.

Cleans up sparse warning:
drivers/gpu/drm/bochs/bochs_mm.c:197:22: warning: symbol 'bochs_bo_driver'
was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180207111353.30261-1-colin.king@canonical.com