drm/omap: Split mode fixup and mode set from encoder enable

The encoder enable operation currently performs mode fixup and mode
setting for all omap_dss_device instances in the display pipeline. There
are dedicated encoder operations for those operations (respectively
.atomic_check() and .mode_set()), but they are not used for this
purpose.

Move the mode fixup code to .atomic_check() and the mode set code
.mode_set() to better fit the KMS model. The bus flags fixup has to
happen at .mode_set() time as there is no place to store the bus flags
in the atomic state structures. This could be solved by extending one of
the state structures, but as the goal is to replace the fixup by direct
usage of bus flags through the driver, that would be pointless.

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>