Previously it was possible to change from a D3D9 to D3D11 display because
both were implemented with a DisplayD3D.  Now that there is a DisplayGL,
the DisplayImpl must be updated while maintaining the same EGLDisplay
value.

Allow the value of Display::mImplementation to be updated when the display
is not already initialized.

BUG=angle:890

Change-Id: Ic18a8a120218cd130a71b9aa044b6ec00006d6a0
Reviewed-on: https://chromium-review.googlesource.com/252250Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>