drm/i915: use struct intel_crtc *crtc at __intel_fbc_update()

This change was part of the commit that makes intel_fbc_update()
receive an intel_crtc as argument instead of dev_priv, but since it
was polluting the diff with too many chunks I decided to move it to
its own commit.

It seems that our developers are favoring having this instead of the
old combination drm_crtc *crtc + intel_crtc *intel_crtc, and on the
mentioned commit we'll get rid of the drm_crtc variable, so let's do
an intermediate commit with the rename, so on the next commit we'll
have just struct intel_crtc *crtc.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446664257-32012-6-git-send-email-paulo.r.zanoni@intel.com
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>