drm: omapdrm: dsi: Combine two commonly used inline functions

The dsi_get_dsidrv_data() and dsi_get_dsidev_from_dssdev() inline
functions convert a struct omap_dss_device pointer to the corresponding
struct platform_device, and a struct platform_device pointer to the
corresponding struct dsi_data. They are nearly always called together
without any use of the intermediate platform_device, so combine them
into a single function.

In the three locations where only dsi_get_dsidrv_data() is used, call
dev_get_drvdata() directly.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>