drm/omap: Store bus flags in the omap_dss_device structure

Source components in the display pipeline need to configure their output
signals polarities and clock driving edge based on the requirements of
the sink component.

Those requirements are currently shared across the whole pipeline in the
flags of a videomode structure, instead of being local to each bus. This
both prevents multiple buses from having different configurations (when
the hardware supports it), and makes it difficult to move from videomode
to drm_display_mode as the latter doesn't contain bus polarities and
clock edge flags.

Add a bus_flags field to the omap_dss_device structure and move the
DISPLAY_FLAGS_DE_(LOW|HIGH), DISPLAY_FLAGS_PIXDATA_(POS|NEG)EDGE and
DISPLAY_FLAGS_SYNC_(POS|NEG)EDGE videomode flags to bus_flags in all
external encoders, connectors and panels. The videomode flags are still
used internally for internal encoders, this will be addressed in a
second step.

The related videomode flags in the default mode of the DVI connector can
simply be dropped, as they are always overridden by the TFP410 driver.
Note that this results in both the DISPLAY_FLAGS_SYNC_POSEDGE and
DISPLAY_FLAGS_SYNC_NEGEDGE flags being set, which is invalid, but only
the former is tested for when programming the DISPC, so the DVI
connector flags are effectively overridden by the TFP410 flags.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>