drm/exynos: Rename display_op power_on to dpms

This patch renames the display_op power_on to dpms to accurately reflect
what the function does.

The side-effect of this patch is that the new hdmi dpms callback is now
invoked twice in the dpms path. This is safe and will be dealt with when
the exynos_drm shim goes away.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>