OMAPDSS: DPI displays: Take care of panel timings in the driver itself

OMAPDSS: DPI displays: Take care of panel timings in the driver itself

The timings maintained in omap_dss_device(dssdev->panel.timings) should be
maintained by the panel driver itself. It's the panel drivers responsibility
to update it if a new set of timings is to be configured. The DPI interface
driver shouldn't be responsible of updating the panel timings, it's responsible
of maintianing it's own copy of timings.

Signed-off-by: Archit Taneja <archit@ti.com>