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

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

Mark functions cirrus_set_start_address(), cirrus_encoder_destroy(),
cirrus_vga_get_modes() and cirrus_connector_best_encoder() as static in
drm/cirrus/cirrus_mode.c because they are not used outside this file.

This eliminates the following warnings in drm/cirrus/cirrus_mode.c:
drivers/gpu/drm/cirrus/cirrus_mode.c:105:6: warning: no previous
prototype for ‘cirrus_set_start_address’ [-Wmissing-prototypes]
drivers/gpu/drm/cirrus/cirrus_mode.c:456:6: warning: no previous
prototype for ‘cirrus_encoder_destroy’ [-Wmissing-prototypes]
drivers/gpu/drm/cirrus/cirrus_mode.c:495:5: warning: no previous
prototype for ‘cirrus_vga_get_modes’ [-Wmissing-prototypes]
drivers/gpu/drm/cirrus/cirrus_mode.c:512:21: warning: no previous
prototype for ‘cirrus_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>