drm: extract drm_atomic_uapi.c

This leaves all the commit/check and state handling in drm_atomic.c,
while pulling all the uapi glue and the huge ioctl itself into a
seprate file.

This seems to almost perfectly split the rather big drm_atomic.c file
into 2 equal sizes.

Also adjust the kerneldoc and type a very terse overview text.

v2: Rebase.

v3: Fix tiny typo.

v4:
- Fixup armada, newly converted atomic driver hooray!
- Fixup msm/dpu1, newly added too.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: intel-gfx@lists.freedesktop.org
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Acked-by: R...