drm/savage: dereferencing an error pointer

A recent cleanup changed the kmalloc() + copy_from_user() to
memdup_user() but the error handling wasn't updated so we might call
kfree(-EFAULT) and crash.

Fixes: a6e3918b ('GPU-DRM-Savage: Use memdup_user() rather than duplicating')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161012062227.GU12841@mwanda