drm/auth: make drm_{set,drop}master_ioctl symmetrical

Currently the ret handling is all over the place - with two redundant
assignments and another one addressed earlier.

Use the exact same flow in both functions.

v2: straighten the code flow, instead of just removing the assignments

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200530124640.4176323-2-emil.l.velikov@gmail.com