OMAPDSS: DPI: Maintain our own timings field in driver data

The DPI driver currently relies on the timings in omap_dss_device struct to
configure the DISPC accordingly. This makes the DPI interface driver dependent
on the omap_dss_device struct.

Make the DPI driver data maintain it's own timings field. The panel driver is
expected to call dpi_set_timings()(renamed to omapdss_dpi_set_timings) to set
these timings before the panel is enabled.

In the set_timings() op, we still ensure that the omap_dss_device timings
(dssdev->panel.timings) are configured. This will later be configured only by
the DPI panel drivers.

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