it OMAPDSS: DPI: use VPLL2 regulator if VDDS_DSI is not found

On Device Tree boot the VDDS_DSI regulator is not linked to
the DPI device so omapfb driver probing fails with:

[    3.186035] OMAPFB: omapfb_probe
[    3.190704] omapdss DPI error: can't get VDDS_DSI regulator
[    3.196594] omapfb omapfb: failed to connect default display
[    3.202667] omapfb omapfb: failed to init overlay connections
[    3.208892] OMAPFB: free_resources
[    3.212493] OMAPFB: free all fbmem
[    3.216735] omapfb omapfb: failed to setup omapfb

As a workaround try to use the VPLL2 regulator from twl4030 in
dpi_init_regulator() if getting the VDDS_DSI regulator fails.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>