drm/exynos: move triggering checking

It's better to be checking whether triggerring in fimd_trigger function.
Also it will return if in triggerring on fimd_te_handler, then it can't
execute remain codes.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>