drm/exynos: added virtual display driver.

this driver would be used for wireless display. virtual display
driver has independent crtc, encoder and connector and to use
this driver, user application should send edid data to this driver
from wireless display.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>