egl: let each platform decided how to handle LIBGL_ALWAYS_SOFTWARE

My refactor in 47273d73 missed this early return; because
of it, setting UseFallback one layer above actually prevented the
software path from being used.

Remove this early return and let each platform's dri2_initialize_*()
decide what it can do with the LIBGL_ALWAYS_SOFTWARE restriction.

platform_{surfaceless,x11,wayland} were already handling it themselves.

Fixes: 47273d73 "egl: set UseFallback if LIBGL_ALWAYS_SOFTWARE is set"
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reported-by: Brendan King <Brendan.King@imgtec.com>
(cherry picked from commit 2f421651)