drm/mgag200: fix error return code in mgag200fb_create()

Fix to return error code -ENOMEM from the vmalloc() error handling
case instead of 0, as done elsewhere in this function.

Fixes: aec9e129 ("drm/mgag200: Fix error handling paths in fbdev driver")
Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1471273431-6753-1-git-send-email-weiyj.lk@gmail.com