drm: include kernel.h and agp_backend.h from intel-gtt.h

intel-gtt.h uses kernel and agp pointers, make it self-contained by
including the relevant headers. This prepares for dropping drmP.h from
files including intel-gtt.h.

[Updated commit message per Laurent's review while applying.]

Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9414800f89d6f326c6e9173233bf6c4dee254d9a.1545915059.git.jani.nikula@intel.com