OMAPDSS: DPI: fix compilation if DSI not compiled in

Commit 100c8262 (OMAPDSS: DPI: use new
clock calculation code) breaks dpi.c compilation if DSI is not enabled
in the kernel configuration.

Fix compilation by adding dummy inline functions for the ones that dpi.c
references. The functions will never be called, as dpi.c knows that
there is no DSI device available.

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