drm/i915: Add main atomic entrypoints (v2)

Add the top-level atomic entrypoints for check/commit.  These won't get
called yet; we still need to either enable the atomic ioctl or switch to
using the non-transitional atomic helpers for legacy operations.

v2:
 - Use plane->pipe rather than plane->possible_crtcs while ensuring that
   only a single CRTC is in use.  Either way will work fine since i915
   drm_plane's are always tied to a single CRTC, but plane->pipe is
   slightly more intuitive. (Ander)
 - Simplify crtc/connector checking logic. (Ander)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>