drm/sde: only set split registers for DSI interfaces

The SPLIT related registers are only for DSI interfaces. Without
checking the interface type, they could be overwrote by
configurations through HDMI path.

CRs-Fixed: 1085586
Change-Id: I7ace9fd8dfe5ee99cb750b2723e8f22701039552
Signed-off-by: Jin Li <jinl@codeaurora.org>