When releasing the thread only call makeCurrent if we have a display
to use. Calling with EGL_NO_DISPLAY is not valid (though some devices
allow it.)

Bug: angleproject:3102

Change-Id: I2cf48011294d0264d3b1cb0318a2a80715e37f60
Reviewed-on: https://chromium-review.googlesource.com/c/1436840Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>