drm/i915: Pass intel state to plane functions as well

Pass along the correct state as much as possible, instead of relying
on the drm state internally. This is required to rely on hw state
internally soon.

While at it, clean up intel_plane_atomic_check slightly, by using a
helper function to get the intel_crtc. (Ville)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190628085517.31886-6-maarten.lankhorst@linux.intel.com