drivers: gpu: Mark function as static and remove unused function in ast_main.c

Mark function ast_bo_unref() as static because it is not used outside
file ast_main.c and remove unused function ast_get_max_dclk() in
ast_main.c.

This eliminates the following warning in drm/ast/ast_main.c:
drivers/gpu/drm/ast/ast_main.c:192:10: warning: no previous prototype
for ‘ast_get_max_dclk’ [-Wmissing-prototypes]
drivers/gpu/drm/ast/ast_main.c:452:6: warning: no previous prototype for
‘ast_bo_unref’ [-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>