drm/ast: Remove set but not used variable 'bo'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/ast/ast_fb.c: In function 'astfb_create':
drivers/gpu/drm/ast/ast_fb.c:194:17: warning:
 variable 'bo' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit 312fec14

 ("drm: Initial KMS
driver for AST (ASpeed Technologies) 2000 series (v2)")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1544258185-50430-1-git-send-email-yuehaibing@huawei.com