ecore-drm: add ecore_drm_devices_get to get the list of drm devices

ecore-drm: add ecore_drm_devices_get to get the list of drm devices

Summary:
    when enlightenment is working as wayland display server, enlightenment
    changes KDSETMODE to KD_GRAPHICS in _ecore_drm_tty_setup(). However,
    when enlightenment is killed by SIGSEGV, it doesn't changes KDSETMODE
    to KD_TEXT because englightenment process doesn't call ecore_drm_tty_close().
    To make possible enlightenment call ecore_drm_tty_close(), drm devices
    should be exposed.
    When enlightenment is killed by SIGSEGV, it will get drm devices with
    ecore_drm_device_get_list(), and will call ecore_drm_launcher_disconnect(),
    and ecore_drm_launcher_disconnect will call ecore_drm_tty_close() internally.

@feature

Reviewers: zmike, cedric, raster, gwanglim, devilhorns

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D2159

Change-Id: Ie32b6eb02831bd95fd23d6a4f1dae372f999f4cc