drm/i915: Share crtc setup and teardown between dpms and disable/enable

This closes a couple of corner cases where we introduced and forgot
about a couple of routines that need to be called when disabling the
crtc and then re-enabling it. The code needs to be moved again so that
the common bits are shared across generations.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_drv.h

@@ -180,7 +180,7 @@ struct intel_crtc {
 	uint32_t cursor_addr;
 	int16_t cursor_x, cursor_y;
 	int16_t cursor_width, cursor_height;
-	bool cursor_visible, cursor_on;
+	bool cursor_visible;
 };
 
 #define to_intel_crtc(x) container_of(x, struct intel_crtc, base)