Support atomic modesetting ioctl

Add support for the atomic modesetting ioctl through a property-set API.

v1: Squashed intermediate patches from Ville, Rob and myself. Updated
    for current kernel interface (no blobs).
v2: Rewrite user-facing API to provide transactional/cursor interface.
    Use memclear to zero out ioctl.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rob Clark <robclark@freedesktop.org>
Signed-off-by: Daniel Stone <daniels@collabora.com>

v3 [Emil Velikov]: Remove DRM_CAP_ATOMIC - superseded by
DRM_CLIENT_CAP_ATOMIC.
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>