drm/bochs: mark bochs_connector_get_modes() static

We get 1 warning when building kernel with W=1:
drivers/gpu/drm/bochs/bochs_kms.c:181:5: warning: no previous prototype for 'bochs_connector_get_modes' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
So this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1474788334-11007-1-git-send-email-baoyou.xie@linaro.org