drm/i915: Nuke fbc members from intel_crtc->atomic, v4.

drm/i915: Nuke fbc members from intel_crtc->atomic, v4.

Whenever there's an update to the primary plane,
fbc_pre_update and fbc_post_update are called. Kill off
intel_crtc->atomic.update_fbc and now that intel_crtc->atomic
is empty, kill it off too.

Changes since v1:
- Add a intel_fbc_supports_rotation helper.
Changes since v2:
- Remove intel_fbc_supports_rotation_helper.
- Remove unrelated changes.
Changes since v3:
- Rebase

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457516145-32117-2-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>