Switch to the preferred 'crtc' name for our crtc variables.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200225171125.28885-2-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>