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

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

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

Make the SDI panel driver configure the new timings is the omap_dss_device
struct(dssdev->panel.timings). The SDI driver is responsible for maintaining
only it's own copy of timings.

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