drm/exynos: page flip fixes

The event wouldn't be on any list at this point, so nothing to delete
it from.

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
drm/exynos: use drm_send_vblank_event() helper

Rebased.

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
drm/exynos: accept flip requests when the crtc is off

Previously, we were rejecting flip requests when off. But
Mali does not like being told 'no' for an answer, and ignores it.
So let's honour these requests as if the display were powered up.