OMAP: DSS2: Check for SDI HW before accessing SDI registers

Only OMAP 3430 hardware has SDI support. The availability of SDI HW can
be found out by checking if the LCD channel supports SDI displays.

This patch checks for SDI HW support before accessing SDI registers,
which fixes a crash on OMAP4 when SDI SW support is compiled in.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>