drm: remove redundant drm_file->uid

drm: remove redundant drm_file->uid

Each DRM file-context caches the EUID of the process that opened the file.
It is used exclusively for debugging purposes in /proc/dri/ and friends.

Note, however, that we can already fetch the EUID from
priv->pid->task->creds. The pointer-chasing will not hurt us, since it is
only about debugging, anyway.

Since we already are in an rcu-read-side, we can use __task_cred() rather
than task_cred_xxx().

Signed-off-by: David Herrmann <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]