egl: refactor dri2_create_screen() into three separate functions

egl: refactor dri2_create_screen() into three separate functions

Split the create_screen into:
 - create screen
 - setup/bind extensions
 - setup screen

This will allow us to reuse the latter two on egl/drm. Said platform
does create its own screen and attempts to reinvent the later two
functions itself.

Since the GBM ones tend to get out of sync quite often, and there is no
distinct reason why it does so we'll drop them with latter commits.

v2: disp -> dpy for the Android platform.
v3: use correct goto label (Rob)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Rob Herring <robh@kernel.org>