Don't Attend/Ignore all clients in driver Enter/Leave VT handlers

DRI2 now does this for us, and does it properly by tracking which
clients are currently using DRI2, and also ignoring any new clients that
arrive while we are currently away on another VT.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>

BUG=chromium:220687
TEST=Install developer image
  Login as guest
  Browse to www.youtube.com
  VT switch
  Wait 15 seconds
  VT switch back
  => User should NOT be logged out!

Change-Id: I850b97d77e9cc4b905dcad95e4fc2c0136a6b466
Reviewed-on: https://gerrit.chromium.org/gerrit/42962
Commit-Queue: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>
1 file changed