drm: delete drmP.h + drm_os_linux.h

There is finally no more users left in the kernel of drmP.h
and drm_os_linux.h (drmP.h was the only user left).
Delete the header files and delete the corresponding todo entry.

When we started this quest there was more than 700 users of drmP.h.
And drmP.h was a huge cover-it-all header file.

Daniel Vetter is the one that followed the work from start
to the end and in between many people have contributed to the
removal process - thanks to everyone!

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Sean Paul <sean@poorly.run>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191007171224.1581-3-sam@ravnborg.org

Documentation/gpu/todo.rst

@@ -299,18 +299,6 @@ connector register/unregister fixes
 Core refactorings
 =================
 
-Clean up the DRM header mess
-----------------------------
-
-The DRM subsystem originally had only one huge global header, ``drmP.h``. This
-is now split up, but many source files still include it. The remaining part of
-the cleanup work here is to replace any ``#include <drm/drmP.h>`` by only the
-headers needed (and fixing up any missing pre-declarations in the headers).
-
-In the end no .c file should need to include ``drmP.h`` anymore.
-
-Contact: Daniel Vetter
-
 Make panic handling work
 ------------------------