drm: return -EFAULT if copy_to_user() fails

The copy_from_user() function returns the number of bytes remaining
to be copied but we want to return a negative error code.  Otherwise
the callers treat it as a successful copy.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618131843.GA29463@mwanda