drm/exynos: do not use generic flags to dumb

This patch removes the use of dumb flags from driver.

As Dave pointed out, the dumb flags are not driver specific
so this should be removed from driver.

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