drm/i915: Add intel_atomic_get_crtc_state() helper function

The pattern of getting the crtc state with drm_atomic_get_crtc_state()
and then converting it to intel_crtc_state will repeat quite often in
the following patches, so add a helper function to save some typing.

v2: Fix upcasting so that crtc_state base field could be moved. (Daniel)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>