drm/doc: Document uapi requirements in DRM

drm/doc: Document uapi requirements in DRM

Everyone knows them, except all the new folks joining from the ARM
side haven't lived through all the pain of the past years and are
entirely surprised when I raise this. Definitely time to document
this.

Last time this was a big discussion was about 6 years ago, when qcom
tried to land a kernel driver without userspace. Dave Airlie made the
rules really clear:

http://airlied.livejournal.com/73115.html

This write-up here is essentially what I've put into a presentation a
while ago, which was also reviewed by Dave:

http://blog.ffwll.ch/2015/05/gfx-kernel-upstreaming-requirements.html

v2: Fix typos Eric&Rob spotted.

v3: Nitpick from Jani.

Cc: Dave Airlie <airlied@gmail.com>
Cc: Oded Gabbay <oded.gabbay@gmail.com>
Cc: Russell King <rmk+kernel@armlinux.org.uk>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Mark Yao <mark.yao@rock-chips.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: CK Hu <ck.hu@mediatek.com>
Cc: Xinliang Liu <z.liuxinliang@hisilicon.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Maxime Ripard  <maxime.ripard@free-electrons.com>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Brian Starkey <brian.starkey@arm.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>