drm/i2c/tda998x: drop use of drmP.h

Drop use of the deprecated drmP.h header file.
Fix fallout.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190804094132.29463-2-sam@ravnborg.org