drm/exynos/dsi: mask frame-done interrupt

DSI driver is not really interested in this interrupt. It causes only
unnecessary code execution of interrupt handler and could possibly
cause FIFO overflow - as it triggers DSI interrupt handler to process
next DSI transfer. With this patch we will get rid of about 30 IRQ
handler calls per second.

Fixes: e6f988a4 ("drm/exynos: dsi: add support for Exynos5433")
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>