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

Mark functions ast_ttm_global_release(), ast_ttm_bo_is_ast_bo() and
ast_ttm_tt_create() as static in drm/ast/ast_ttm.c because they are not
used outside this file.

This eliminates the following warnings in drm/ast/ast_ttm.c:
drivers/gpu/drm/ast/ast_ttm.c:84:1: warning: no previous prototype for
‘ast_ttm_global_release’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_ttm.c:105:6: warning: no previous prototype for
‘ast_ttm_bo_is_ast_bo’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_ttm.c:211:16: warning: no previous prototype for
‘ast_ttm_tt_create’ [-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>