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

Mark functions ast_set_sync_reg(), ast_set_dac_reg(),
ast_set_start_address_crt1(), ast_crtc_init(), ast_encoder_init(),
ast_connector_init(), ast_cursor_init(), ast_cursor_fini(),
ast_show_cursor() and ast_hide_cursor() as static in drm/ast/ast_mode.c
because they are not used outside this file.

This eliminates the following warning in drm/ast/ast_mode.c:
drivers/gpu/drm/ast/ast_mode.c:407:6: warning: no previous prototype for
‘ast_set_sync_reg’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_mode.c:418:6: warning: no previous prototype for
‘ast_set_dac_reg’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_mode.c:430:6: warning: no previous prototype for
‘ast_set_start_address_crt1’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_mode.c:626:5: warning: no previous prototype for
‘ast_crtc_init’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_mode.c:713:5: warning: no previous prototype for
‘ast_encoder_init’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_mode.c:780:5: warning: no previous prototype for
‘ast_connector_init’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_mode.c:813:5: warning: no previous prototype for
‘ast_cursor_init’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_mode.c:850:6: warning: no previous prototype for
‘ast_cursor_fini’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_mode.c:968:6: warning: no previous prototype for
‘ast_show_cursor’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_mode.c:979:6: warning: no previous prototype for
‘ast_hide_cursor’ [-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>