drm/exynos: remove dead code in vidi_power_on

The type of input parameter enable is bool, so it does not need
to check whether true or false.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>