egl: Simplify the "driver" interface

egl: Simplify the "driver" interface

"Driver" isn't a great word for what this layer is, it's effectively a
build-time choice about what OS you're targeting. Despite that both of
the extant backends totally ignore the display argument, the old code
would only set up the backend relative to a display.

That causes problems! One problem is it means eglGetProcAddress can
generate X or Wayland protocol when it tries to connect to a default
display so it can call into the backend, which is, you know, completely
bonkers. Any other EGL API that doesn't reference a display, like
EGL_EXT_device_query, would have the same issue.

Fortunately this is a problem that can be solved with the delete key.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>