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

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

Make the VENC driver data maintain it's own timings field. The panel driver is
expected to call omapdss_venc_set_timings() to set these timings before the
panel is enabled. Call omapdss_venc_set_timings() before enabling
venc output, this is done to atleast have the venc output configured to the
panel's default timings if the DSS user didn't explicitly call the venc panel
driver's set_timings op.

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

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