OMAPDSS: DPI: fix crash with dpi_verify_dsi_pll()

If the DSI support has not been compiled in or the SoC doesn't have DSI
hardware, dpi_get_dsidev() returns NULL. This NULL is passed to
dpi_verify_dsi_pll() causing a crash. The bug was added with commit
0e8276ef (OMAPDSS: DPI: always use DSI
PLL if available).

Fix this by checking if dsidev is NULL before calling
dpi_verify_dsi_pll().

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