Drop remaining uses of the deprecated drmP.h in gma500

Replaced drmp.h with forward declarations or include files
as relevant.

Moved all include files to blocks in following order:
\#include <linux/*>

\#include <asm/*>

\#include <drm/*>

\#include ""

And within each block sort the include files alphabetically.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190519195526.3422-6-sam@ravnborg.org