egl: Allow a prioritized list of default drivers

egl: Allow a prioritized list of default drivers

When there is no user driver or any matching display drivers we fall
back to the default driver.  This patch lets us have a list of default
drivers instead of just one.  The drivers are loaded in turn and we
attempt to initialize the display.  If it fails we unload the driver
and move on to the next one.

Compared to the display driver mechanism, this avoids loading a number
of drivers and then only using one.  Also, we call Initialize to see
if the driver will work instead of relying on Probe.  To know for sure
that a driver will work, Probe really have to do a full Initialize, so
we will just use Initialize directly.