OMAPDSS: fix TV-out issue with DSI PLL

OMAPDSS: fix TV-out issue with DSI PLL

Commit 09855ccd47459c9e0de0ee3db61d7baa3403f6c6 (OMAPDSS: DPI: always
use DSI PLL if available) made dpi.c use DSI PLL for its clock. This
works fine, for DPI, but has a nasty side effect on OMAP3:

On OMAP3 the same clock is used for DISPC fclk and LCD output. Thus,
after the above patch, DSI PLL is used for DISPC and LCD output. If
TV-out is used, the TV-out needs DISPC. And if DPI is turned off, the
DSI PLL is also turned off, disabling DISPC.

For this to work, we'd need proper DSS internal clock handling, with
refcounts, which is a non-trivial project.

This patch fixes the issue for now by disabling the use of DSI PLL for
DPI on OMAP3.

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