drm/omap: DPI: rename "DSI" related names

Originally DSS only had DSI PLLs, and thus the DPI driver has functions
and variables that refer to DSI or DSI PLL. Now we support DSI, VIDEO
and HDMI PLLs, so it's time to remove the DSI references from the code.

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