drm_hdcp.h uses kernel integer types, make it self-contained by
including linux/kernel.h. This prepares for dropping drmP.h from files
including drm_hdcp.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>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/adfc79941e827b49eb6186f317cbeff0fb365f33.1545915059.git.jani.nikula@intel.com