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