egl: extract EGLDevice setup in dedicated function

egl: extract EGLDevice setup in dedicated function

Extract the logic responsible for populating disp->Device via
_eglFindDevice(). This isn't much for now but will grow in a
following commit.

No functional changes.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Tested-by: Iago Toral Quiroga <itoral@igalia.com>
Tested-by: Alejandro Piñeiro <apinheiro@igalia.com>
Backport-to: 23.3
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26205>
(cherry picked from commit b4ec0b51ce47d87d21609444dddd50c91968f26d)