drm/i915: s/intel_crtc/crtc/ in .crtc_enable() and .crtc_disable()

Get rid of the horrible aliasing drm_crtc and intel_crtc variables
in the crtc enable/disable hooks.
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118164430.27265-9-ville.syrjala@linux.intel.com