OMAPDSS: SDI: Maintain copy of data pairs in driver data

The SDI driver currently relies on the omap_dss_device struct to configure the
number of data pairs as specified by the panel. This makes the SDI interface
driver dependent on the omap_dss_device struct.

Make the SDI driver data maintain it's own data lines field. A panel driver
is expected to call omapdss_sdi_set_datapairs() before enabling the interface.
Even though we configure the number of data pairs here, this function would be
finally mapped to a generic interface op called set_data_lines. The datapairs
argument type has been changed from u8 to int at some places to be in sync with
the 'set_data_lines' ops of other interfaces.

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