egl: Rename _eglAddDevice() to _eglFindDevice()

egl: Rename _eglAddDevice() to _eglFindDevice()

_eglAddDevice() is now only used to look up eglDevices,
which means that the function name is a bit misleading.

Since this function is only used for finding the
eglDevice, it should also no longer support adding
a device to the egl devices list.

Signed-off-by: Robert Foss <rfoss@kernel.org>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23742>