drm: rcar-du: Remove inclusion of drmP.h

The DRM kernel API used to be defined in a handful of headers, pulled in
through drmP.h. It has since been split in multiple headers for the
different DRM components, and drmP.h turned into a legacy header that
just pulls in most of the DRM kernel API (and a large number of other
miscellaneous kernel headers).

In order to speed up compilation, replace inclusion of drmP.h with only
the required headers. It turns out that the rcar-du-drm driver already
includes most of the necessary headers, so the change is simple.

While at it, remove unneeded inclusion of other headers, and unneeded
forward declarations of structures.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>