drm: Fix locking gotcha in page_flip ioctl

We want to lock the primary plane, not the cursor (which might be
optional). Real bad case of copy-paste fail, unfortunately our CI
didn't catch that because i915 does have a cursor plane.

Reported-by: Eric Anholt <eric@anholt.net>
Fixes: 29dc0d1d

 ("drm: Roll out acquire context for the page_flip ioctl")
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170330204831.8225-1-daniel.vetter@ffwll.ch
Tested-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>