drivers: gpu: Mark functions as static in mgag200_mode.c

Mark functions mga_set_start_address(), mga_encoder_destroy() and
mga_connector_best_encoder() as static in drm/mgag200/mgag200_mode.c
because they are not used outside this file.

This eliminates the following warnings in drm/mgag200/mgag200_mode.c:
drivers/gpu/drm/mgag200/mgag200_mode.c:694:6: warning: no previous
prototype for ‘mga_set_start_address’ [-Wmissing-prototypes]
drivers/gpu/drm/mgag200/mgag200_mode.c:1401:6: warning: no previous
prototype for ‘mga_encoder_destroy’ [-Wmissing-prototypes]
drivers/gpu/drm/mgag200/mgag200_mode.c:1561:21: warning: no previous
prototype for ‘mga_connector_best_encoder’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>