drm/exynos: fimd: only finish update if START == START_S

fimd_update_plane() programs BUF_START[win] and during the update
BUF_START[win] is copied to BUF_START_S[win] (its shadow register)
and starts scanning out, then it raises a irq.

The fimd_irq_handler, in the case we have a pending_fb, will check
the fb value was copied to START_S register and finish the update
in case of success.

Based on patch from Daniel Kurtz <djkurtz@chromium.org>

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