drm/exynos: fimd: unify call to exynos_drm_crtc_finish_pageflip()

Unify handling of finished plane update to prepare for a following patch
that will check for the START and START_S regs to really make sure that
the plane was updated.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>