egl: fix software flag in _eglAddDevice call on DRM

egl: fix software flag in _eglAddDevice call on DRM

On the EGL DRM platform, call _eglAddDevice with the software flag
set if GBM has loaded a software driver. This allows _eglAddDevice
to make the difference between llvmpipe and kmsro.

This is important on split render/display SoCs: we don't want to
advertise EGL_MESA_device_software on these systems.

Completely drop disp->Options.ForceSoftware, because GBM is
responsible for choosing software rendering and doesn't take this
hint into account.

Signed-off-by: Simon Ser <contact@emersion.fr>
Fixes: 5743a36b2b32 ("egl: Don't add hardware device if there is no render node v2.")
References: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4178
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9697>