drm: omapdrm: dsi: Use dev pointer directly in dsi_bind() function

The dsi_bind() function receives a pointer to a struct device that it
casts to a struct platform_device, only to use the platform device's dev
field through the code. Use the dev pointer directly.

While at it rename the struct platform_device pointer dsidev to pdev to
make it more explicit.

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